The TARDIS materialises in mid air and spins downward into the roiling sea. The Doctor, Jamie, and Victoria are forced to use a rubber raft to reach land - a rocky beach. The weather is cold and they are dressed for it. Jamie deduces that it is England, just from the weather alone. The Doctor agrees, due to the geography. Victoria notes how strange it is they keep on landing on Earth, in England, a similar sentiment expressed by the Doctor on their previous landing.

The Doctor's attention is drawn by a mass of foam at his feet. With it is some seaweed that the Doctor discounts. The foam appears to line the shore as far as they can see. Sea foam is common enough, but not in such large quantities. He picks up some and is intrigued by the texture. However, he's not so intrigued that he can't have some fun, spattering Jamie with the foam! Suddenly a foam fight breaks out amongst the three of them.

Their battle moves them inland a bit and Jamie is first to notice a giant pipe coming out of the sea. They break off the fight and go to investigate. Unbeknownst to them, they are being observed... through the sights of a gun!

The pipeline is massive and says "Euro Sea Gas" on it. The Doctor explains it is for extracting gas from the sea and Jamie is sceptical. Victoria wonders if it might have something to do with the foam, but the Doctor doubts it. On top of the pipe is a control box and the Doctor has a go at opening it. He produces a small pen-like device which he calls a "sonic screwdriver" and applies it to the screws on the cover of the box. They begin unscrewing themselves of their own accord. To his companions, it looks like magic, but they've come to expect such things from the Doctor. He says it's all done by sound waves.

The lid comes off and the Doctor inspects the mechanicals inside. However, it is something he hears that really draws his interest. A strange distant noise. The Doctor produces a stethoscope and listens more closely. His first impression is that it is the sound of a heart beating. Victoria agrees as it gets louder and she begins to get nervous.

The Doctor amends his previous analysis and thinks the noise is just from a pump somewhere on the pipeline, but he understands Victoria's nervousness. She is sure they're being watched and wants to go back to the TARDIS. The Doctor agrees and they turn to go. Suddenly a shot rings out and Jamie falls. The Doctor is next, and then the horrified Victoria.

However, they are not dead. They have only been tranquilised. Some time later, they all wake up inside a metal-walled control room. They are having a lot of trouble moving, as if they've been paralysed. It takes a moment for them to adjust to what has happened, but they soon notice the armed guards standing over them. No one will answer the Doctor's questions and tell them where they are.

Shortly, they are joined by two men. One is clearly in charge here, the other an assistant. The leader is much more interested in asking questions than in answering them. But in their present state, the Doctor assures him they can be of very little help. The leader relents and orders his assistant, Mr. Harris, to administer U4, an antidote to the tranquiliser. It works quickly and the three prisoners are soon on their feet.

The leader tells the Doctor that they were in a restricted area of the pipeline, tampering with the emergency release valve remote control. He accuses the Doctor of being a saboteur. The Doctor and his friends deny the charge, saying they were only lost and curious. Harris tries to step in, seeming to believe their story, but the high-strung leader cuts him off. He has no interest in Harris' opinions or the prisoners' story. He orders them locked up in a cabin until he is ready to talk with them again and then gets back to his work.

Harris seems somewhat embarrassed of his leader's behaviour and tries to explain to the Doctor what is going on. They are in a bit of an emergency just now. They have lost contact with one of their gas drilling rigs at sea - not a communications breakdown but a seeming refusal on the part of the crew to respond to calls. On top of that, there has been a pressure drop in the feed line from all the rigs. The appearance of these strangers in a restricted area, near the release valve, is very suspicious indeed. Harris himself doesn't appear to suspect them of foul play, but the leader, Mr. Robson, is desperate to find the cause. He is under "considerable pressure" to solve the problem.

Reluctantly, Harris leads them off to lock them up. Robson's orders.

Elsewhere in the control compound, the emergency has other guards on high alert. A woman tries to out of the main control compound without a pass. Even though the guard knows her - Maggie Harris, Mr. Harris' wife - he will not let her in. Robson's orders. He suggests she return to the residential block.

Harris leads his prisoners to a cabin where they are to be locked up. The compassionate man tries one more time to find out why the Doctor was tampering with the release valve. The Doctor assures him it wasn't the valve he was interested in, it was the sound that he heard inside. Some sort of "movement" in the pipeline. Harris is confused, certain that there is no way that marine life could get inside the pipe, despite Jamie and Victoria's support of the Doctor. They all heard something. The Doctor suggests that there might be some kind of fracture in the pipe somewhere that has allowed some marine life in...and been responsible for the drop in pressure. Harris says this is impossible - the party line from Robson - but his voice seems to admit the possibility. The Doctor suggests shutting off the gas flow until it can be investigated but Harris says that Robson would never agree to it. He is a proud man, always a stickler for the rules. The gas flow has never been shut off since he's been in charge, and he will not let it happen now.

The Doctor thinks that a very silly philosophy, and a dangerous one. Harris wonders how the Doctor appears to know so much about the gas drilling business, for one supposedly so innocent.

In the control room, contact has at last been established with the formerly silent Rig D. The face of a man called Carney appears on the monitor screen as Robson bellows questions. Carney is strangely calm and speaks only in a whisper. Nothing conspiratorial, just quiet and sibilant. Carney says that everything is quite all right and that the "situation" is under control.

This is not enough for Robson who demands answers. He learns that the emergency crew sent to the rig did indeed arrive but that they "must stay here for the time being". Robson becomes angrier by the second at these cryptic non-answers. Carney adds that they had a "slight accident" and that two men are out of action. It will take some time to fix the problems on the rig, although he doesn't say what those problems are. Carney then continues to repeat the phrase "everything is under control" until the link is lost.

Robson is furious and orders the communications officer - Price - to try to reestablish the link. Still seething, Robson is in no mood when Harris arrives saying that they should shut down the gas flow on the pipeline to make a check of the problems. He tells Robson what the Doctor heard. As expected, Robson scoffs.

They are joined by the Chief Engineer, a no-nonsense man of numbers and figures and machines. He reports a further drop in pressure at propeller number 3. The pressure is now dropping 3% every 30 minutes, an unacceptable situation for Robson. Harris thinks this proves that there must be something blocking the pipeline, but Robson dismisses this. Probably just a faulty gauge. He sends the Chief off to investigate.

Harris continues his suggestion of something blocking the pipe line, continuing to irritate Robson. It is clear that Robson thinks Harris a know-nothing bureaucrat with very little practical experience of the rigs. Robson has spent his entire career knee-deep in the hardware and the daily grind of this work and does not appreciate this upstart coming in to tell him his job. To his credit, Harris doesn't back down. He explains that it's not just what the Doctor has said. It all fits in with what he's observed and recorded over the last three weeks. There has been a regular and increasing build up in pressure falls, corresponding to a recognisable pattern that he has documented.

Robson is certain that Harris is trying to get him fired or at least prove that he knows his job better, but Harris persists that he is only doing his own job. Whether Robson takes notice of it or not is up to him. But to do his job, Harris must at least make him aware of his findings. He goes to his desk to get the file of information he's collected.

The Chief returns having found no sign of faulty gauges. He and Robson talk whilst Harris begins ransacking his desk. Somehow, the file has gone. Very strange. Harris thinks he must have left it at home and turns to go and get it. At first, Robson refuses to allow him to leave. They are in an emergency situation and he must stay here. However, when Harris accuses him of being afraid of what the file may show, Robson relents. He sends Harris away to fetch his little file.

However, he reminds Harris - in a deadly calm voice - that he had better come back with something good. Otherwise, he'll chew up and spit out the little university-trained hack. The threat - and Robson's contempt - is very clear.

In their cabin, the Doctor, Jamie, and Victoria are working to escape. Jamie is perched on the Doctor's shoulders, working to prize open a grille above the door. Victoria thinks this all a waste of time, suggesting they try to pick the lock with a hair pin. Jamie dismisses this. Seeing no one about, Jamie sets to work on the grille. It gives easily - too easily - and clatters to the floor with a very loud noise. Not very stealthy at all.

On his way to the residential block, Harris runs into his wife in the corridor. She is angry and tells him what has happened. She was on her way to meet him to get a pass to go out. Harris doesn't have time to get her the pass due to the emergency. He asks her instead to return to their quarters and try and find his file on the pressure drops. He tells her right where it is and she agrees to go, but Maggie is worried about the seeming panic in her husband's manner. It is not like him to be so flustered. He tells her he'll explain later, once she's found the file. She heads off whilst Harris returns to the control room.

Jamie has managed to squeeze into the grille area but is stuck there. Victoria finds this mildly amusing, all the while working at the door with her hair pin. With the Doctor's help, Jamie finally squeezes through, but the Doctor pushes too hard and Jamie falls gracelessly to the floor on the other side.

As he picks himself up, Victoria succeeds in picking the lock. The door opens and she is triumphal. The three of them head outside.

Maggie returns to the apartment and quickly finds the file her husband is looking for. She goes to grab it, only to find a clump of seaweed in the desk. She snatches her hand back, feeling as though she's been stung by the stuff. She grabs it and tosses it away, rubbing her painful hand.

Unseen, the seaweed begins to bubble and seethe, a small mass of foam surrounding it.

In the control room, Robson has yet another problem to deal with. A headstrong Dutchman named Van Lutyens has returned from the Control Rig with grave news. He says that the men there are very restless and worried due to the loss of contact with Rig D. Robson dismisses his concerns, telling him that they've already regained contact with the rig. He conveniently forgets to mention the oddness of that contact and the strange message from Carney.

Van Lutyens insists that the men are nervous over these unexplained incidents and that it is in Robson's interests to keep them informed of developments. The silence only makes their isolation on the rigs that much more difficult. Like Harris, Van Lutyens does not have Robson's respect. He is here on sufferance only on behalf of the Dutch government. It was not his choice to have the man here, he was forced on him by the board of Euro Sea Gas. Worker morale is Robson's problem, not Van Lutyens and he would like it very much if Van Lutyens would mind his own business.

Robson explains that he spent four long years working on the rigs without a break. Only he knows what it's like. Van Lutyens is only a functionary of the Dutch government interest in the project. His "advice" means nothing. Thoroughly insulted, Van Lutyens stalks off, muttering in Dutch.

Meanwhile, the Doctor, Jamie, and Victoria have made it to the control room but are concealed in a glassed-in anteroom. They watch and listen.

A call comes in from Chief Baxter on the Control Rig. He wishes to speak to Robson, hoping he has heard from and will respond to Van Lutyens' information. Robson again dismisses the concern, noting that he is in charge of this operation, not Van Lutyens.

However, Baxter has called for another reason. He says that "something" has gotten into the pipe line. All of his men have heard the same sound. A sort of "heart beat" pulsating.

The Doctor, overhearing this, becomes more concerned than ever. It is the same noise he heard on the beach. He's still not sure what it means, but he wants to find out. He and Jamie head off to investigate, sending Victoria back to the cabin to wait for them in safety.

Once they are out of sight, Victoria disobeys the order and goes off in the opposite direction. The adventuresome girl will do some investigating on her own.

Maggie Harris, not looking at all well now, contacts the control room via her communications unit. She wants to speak to her husband. While she waits, she rubs her injured hand.

Harris, having received the message, tries to get permission from Robson to go to her. Seeing this as a sign of weakness, Robson denies the request. This of course also stops Harris from fetching his precious file. He tells Harris to call a doctor instead.

However, the base's doctor, Patterson, is still with the emergency crew on Rig D and cannot be reached. And because Robson has closed off the base due to the emergency, they cannot get a doctor from the nearby village as well. Angrily, Robson agrees to let Harris go to her. But he'd better make it quick.

Victoria creeps down the hallway, now not sure at all that she has done the right thing. She's not even sure what she's looking for and could be recaptured at any moment. Hearing footsteps approaching, she ducks into the nearest room - the oxygen store - and closes the door.

Once inside, she hears a strange hissing sound and goes toward it. One of the many oxygen canisters stored here has been opened and is leaking its contents into the room. Eventually the hissing stops as the cylinder empties, but Victoria cannot understand how it came to be opened in the first place.

Suddenly there is a movement and she sees a tall man in a white jumpsuit leaving the room. Outside, he quickly locks the door and begins to adjust the environmental controls on the wall.

The Doctor and Jamie reach the impeller area, the massive pump that controls the flow of gas along the entire pipeline network. Jamie has never seen anything like it and finds it hard to believe this is a continuation of the pipe they saw on the beach. Off the impeller area is a pipe room. Oddly, one section of the many pipes there is transparent. The Doctor says it is for checking condensation and "anything else" that might get into the pipes.

Victoria finds that she is now firmly locked in the oxygen store. She bangs on the door and calls out, hoping it's all been a mistake, but there is no response. Suddenly, from the vents in the room, foam begins pouring inside.

In the impeller area, the heartbeat sound begins issuing from all around. The Doctor and Jamie recognise it as the same sound they heard on the beach. They cannot figure out what is causing it, but there is clearly something inside the pipes.

But they soon hear a more important sound - Victoria's voice. She is calling out for them and sounding rather desperate. Jamie and the Doctor rush out to find her.

More and more foam pours from the vents into the little store room. But it is more than foam. There is a mass of seaweed with it. One seems to feed off the other and it grows together, hissing menacingly. It appears to be after Victoria and there is nowhere for her to go. She backs away and calls out for her friends.

The Doctor and Jamie return her calls, desperate to find her.

Victoria is wide eyed with terror now as she sees the seaweed take on a shape. It looks like a solider creature, a mass of seaweed rising out of the surge of foam. She has backed against the door and watches helplessly. She calls out for her friends one last time, but there is nothing she can do.

The Doctor and Jamie follow the screams and reach the oxygen store room. The door is locked, but from beneath it seeps some kind of toxic gas. They are both overcome with sneezing fits and work hard to compose themselves. Their friend is still trapped inside.

They force the door open and manage to drag Victoria out. They slam the door shut and collapse against it, struggling for breath.

Robson, hearing the commotion, arrives in the corridor with Van Lutyens on his heels. Robson is angered that they are free. Van Lutyens opens the store room door and the gas is released again, nearly choking them all. Van Lutyens covers his mouth with a handkerchief and goes inside to investigate.

Victoria is calmer now, but she is still horrified by her experience. She tells the Doctor of the seaweed "creature" she saw - the foam and weed and hissing. It was horrible. The Doctor comforts the sobbing girl and Robson dismisses her story as hysteria.

Van Lutyens returns with no news of a creature, but he has discovered that all of the oxygen bottles have been tampered with, their contents emptied. Robson immediately accuses Victoria of breaking into the locked room and sabotaging the oxygen cylinders. She denies this, saying she hid in there and that the door was unlocked when she tried it. Then she was locked inside by someone. This makes sense to everyone but Robson, who is determined to expose these strangers as saboteurs.

Van Lutyens reminds Robson that the room was not full of oxygen but of some toxic gas, despite the empty cylinders. The Doctor notes the open ventilator. Clearly whoever locked Victoria inside opened the ventilator in order to pump in the foam and gas. Van Lutyens notices the environmental controls in the "open" position and it all makes sense.

Robson, however, is still sceptical.

In the Harris' quarters, Mr. Harris tends to his ill wife. She is feeling dazed and is breathing very heavily. She explains again about the seaweed that "stung" her hand, thinking that is the cause, but Harris can find no evidence of a mark. He tries to calm her, offering to fix her some food or a drink. He is doing his best to offer her comfort, even though he is a better scientist than a house husband.

For a moment, Maggie seems calm but suddenly sits up and hugs her husband, clinging tightly to him. She is wild-eyed. Outside, where she tosses the clump of seaweed, it has grown from a small piece to a much larger one, covered in foam. It hisses the toxic gas and seems to emit a thumping heartbeat sound. The noise burns into Maggie's mind, even though Harris cannot seem to hear it. The intensity is too much and Maggie slumps back on the bed. She is drained by the experience and closes her eyes.

Harris decides he must try and find a doctor and hurries off, assuring his wife he won't be long. As soon as he is gone, the heartbeat sound returns. Maggie opens her eyes and rises from the bed. She is almost zombie-like, her eyes blank.

The sound pulls her into the kitchen area, toward the outside door. It is the weed, calling to her. She reaches the open door and suddenly comes to herself again. She slams the door closed and locks it, leaning hard against it. What is happening to her?

Robson and Van Lutyens return to the impeller area to find yet another flap on. The Chief Engineer reports that the impeller pump has slowed down, not even holding the gas flow in check. It is a potential disaster and Robson orders the Chief to do a full and immediate check of the equipment.

The Doctor reports the noise and movement he and Jamie heard in the pipe room earlier. It was the same thumping sound that he heard earlier, the same as was heard by the men on the Control Rig. To Van Lutyens, this is confirmation that there is something in the pipe line. Robson angrily dismisses this, saying that it was a mechanical fault, and he will listen to nothing that contradicts him. He knows he is right.

The Doctor insists it was no mechanical sound he heard and suggests Robson shut down the gas flow until he's had a chance to investigate. Again, the answer is no, despite Van Lutyens' support for the Doctor's plan.

The Chief reports that revs are down another half and Robson urges him to check again for a mechanical fault. Van Lutyens reminds him that without the impeller, gas pressure in the pipeline will cause the whole thing to explode, taking all the rigs with it and he and the Doctor urge him once again to shut down the gas flow. Beleaguered but not bowing, Robson wants some proof. What could possibly be in the pipeline? One of the ridiculous "seaweed creatures" that Victoria supposedly saw? The Doctor admits this is possible but Robson thinks it ludicrous. Without proof, he will not act.

Meanwhile, Jamie and Victoria have been put in the charge of communications officer Price. He is showing them around the control area and explaining how the whole complex works, including the mass of security cameras. He tells them that this complex supplies all the gas for the south of England and Wales and shows them a map of the rigs out at sea. The outlying rigs drill for the gas and pump it to the Control Rig, the "nerve centre" of the operation. From there it comes to the control complex via the main pipeline.

Victoria thinks it very awful and lonely to have to live out at sea on one of those rigs, but Price thinks it can't be that bad. Robson once lived and worked on one for four years straight without ever coming ashore. Jamie thinks that accounts for quite a lot of the acerbic leader's personality flaws. Price disagrees, saying that Robson's all right. And he certainly knows more about the rigs than anyone else here.

Harris, very worried, approaches Price wanting to know if Dr. Patterson has returned from Rig D yet. He has not and Harris decides to turn to the Doctor. He doesn't know that he isn't a medical doctor and he seems quite desperate. Jamie points him out and Harris goes to him in the impeller area, hoping to get him to come and look at Maggie.

Robson reminds him that the Doctor and his friends are still in custody and refuses to let him go. Harris begins to plead but Robson simply seizes on the weakness that he is showing. He won't let anyone's personal life interfere with the work of this complex.

Harris suddenly snaps, his voice low and menacing. It is Robson's fault that there are no other doctor's available. If anything happens to Harris' wife, it will be on Robson's head. The threat is clear and Robson relents. The Doctor has one hour.

Relieved, Harris takes the Doctor, Jamie, and Victoria off to the residential block.

In her quarters, Maggie Harris is visited by two very odd men. One is short and round, the other thin and quite tall. They are all dressed in white, including caps and gloves. Only the short one speaks. His name is Mr. Oak, the other Mr. Quill. Oak identifies them as maintenance controllers, here to see Mr. Harris. He asked them out to make an inspection in the kitchen. Maggie tries to put them off, saying that her husband is not here and that she is not feeling well. However, it cannot wait, under instructions from Robson.

With no other choice, Maggie lets them in. She retires to her room to lie down again, still quite dazed. Once she is gone, the pleasant smiles leave the faces of the men. They head for the kitchen, specifically the cooker, and produce a bag of tools.

The Chief Engineer has completed his inspection of the impeller and reports to Van Lutyens. There is an excessive pressure build up in the main line coming from the Control Rig. If it gets any worse, the entire pipe line will blow wide open. The situation is serious and all evidence points to something interfering with the impeller itself.

Robson gets the same report from the Control Rig and Van Lutyens is sure that this will finally convince Robson to shut off the gas to forestall an explosion. Robson denies this and refuses to shut off the gas. Van Lutyens is certain he's condemning them all to death, but Robson has a plan. He orders one of the release valves open at full pressure. Van Lutyens had already thought of this but is certain that they cannot vent enough pressure in time to prevent an explosion. Robson scoffs at the Dutchman's timidity, confident that his plan will work.

Maggie sits alone in her room as the work continues in the kitchen. She does not know nor care what the workmen are doing. She gazes blankly into her mirror as the heartbeat sound starts up again, boring into her mind.

Out in the kitchen, Oak and Quill complete their "work" and open up the patio door. The cooker hisses gas into the apartment and the seaweed and foam sweep inside from the patio. It is larger than ever now.

Oak and Quill open the bedroom door and stare silently at Maggie. She asks what they're doing but they remain silent. They open their mouths wide and suddenly toxic gas begins hissing from them. She is trapped in the tiny bedroom, which soon fills with toxic gas. She is overcome by the gas and slumps to the floor.

Miraculously, Robson's plan works. The release valve vents the pressure into the air along the beach and the pressure in the pipeline goes down to safety levels. A cheer goes up from the men in the control room. Van Lutyens admits he did not think Robson would succeed, to which the arrogant leader responds with a withering comeback. He is in charge. And he knows best. Not some snot-nosed engineer from the Hague.

However, Van Lutyens reports that the feed out to the receiving stations is still dropping. The impeller is still slowing down. Robson snaps angrily, demanding to know why Van Lutyens and Harris have been trying to tell him his job ever since they arrived. He knows more about drilling for gas in the North Sea than these university-trained engineers will ever know.

On top of this, Price reports that they now cannot raise Rig C on the communications equipment. Van Lutyens takes this as an indication that Robson is far from in control of the situation.

Harris and the others arrive at his quarters to find the door wide open. There is the smell of toxic gas issuing out and they hurry inside. Jamie finds Maggie collapsed on the bedroom floor and they all rush to her side. The Doctor is desperate to get some fresh air inside and instructs Jamie to smash open the bedroom window. He does so as Harris tries to wake up his wife.

There is still no contact with Rig C and Van Lutyens is using this fact to continue his argument with Robson. These mysterious communications failures, inexplicable pressure variations for weeks, everything points to something very problematic in the system and yet Robson will not listen to his advisers or take any action at all. He finds it unconscionable. Robson is furious at this, feeling that Van Lutyens and Harris have been conspiring behind his back. He regards Harris as an upstart college kid with his graph paper and slide rule and he derides Van Lutyens as being an "expert" with no practical experience of the rigs. Robson himself knows every nut and bolt on one of those rigs. He is in charge and will do as he sees fit!

The Chief Engineer calls out to them, indicating another serious slowdown in the impeller. It is down to 140 revs and must be jammed at the base. As they all watch helplessly, the impeller slows and stops. There is an unaccustomed, almost deafening silence in the impeller area. All eyes are on Robson to solve the problem quickly, but the Chief Engineer's ears are drawn to something else - a resonant thumping sound coming from the impeller shaft. The heartbeat sound, now audible to them all.

The Doctor deduces that Maggie is in some sort of coma, possibly from the effects of the toxic gas. Harris is confused, thinking it is something to do with the natural gas that powers the heat in the base, but the Doctor tells him it is something different all together. The same gas that was present when Victoria was attacked. Harris explains about Maggie being "stung" by some seaweed but the Doctor can find no marks on her. It is all very strange.

Victoria notices a clump of seaweed nearby and draws the Doctor's attention. Harris reaches out to touch it but the Doctor stops him. He's not sure what is happening but it seems that someone deliberately placed that seaweed in Harris' study so that someone would get stung by it. Harris finds this odd. He is now certain that he brought the file with him to work in the morning and he cannot understand how it ended up back at his home with seaweed on it.

Jamie cannot understand all this fuss about seaweed. He remembers plenty of it on the beach around the pipeline earlier but no one said anything then. The Doctor doesn't remember this, but Victoria does. She notes that the stuff on the beach was very different than what she saw in the oxygen store, and the stuff with them right now. It didn't move!

Chaos reigns in the impeller area as technicians and engineers rush about trying to restart the giant impeller. Van Lutyens calls aside the Chief Engineer to speak with him about the strange noise they heard earlier. Robson has dismissed it as a mechanical fault and the Chief is inclined to agree, especially as it hasn't been heard again. Van Lutyens erupts at this, wondering how a seasoned Chief Engineer can so easily forget what he has heard and accept the word of his leader. He accuses the Chief of being scared of Robson. The Chief replies smoothly that he trusts his leader's judgment.

Realising that he may have overstepped his bounds, Van Lutyens backs off a bit and asks to speak to the Chief privately. They move away from the chaos of the impeller area to talk. Van Lutyens explains that he's been looking at the specs for the impeller and he thinks he knows where the blockage must be. He is sure it is at the base of the impeller shaft, near the intake valve, but the Chief is sceptical. Van Lutyens is determined to get down there and free the blockage, but the Chief wants to consult with Robson first.

Van Lutyens explodes once again, accusing the Chief of being unable to do anything without Robson's approval. This provokes a stern and unequivocal response from the Chief: he has worked with Robson for a long time, both here and out on the rigs. His way of working might be a bit abrasive but he knows what he's talking about. The Chief is not afraid of Robson but he trusts the man's judgment. Van Lutyens backs off once again but hopes his theory has at least made some impact on the Chief. He suggests the Chief go to Robson and ask him for permission to go down into the impeller shaft to investigate the blockage. The Chief is resistant at first, but the possibility does make sense and he agrees to go.

Just then Robson bursts into the impeller area, looking for the Chief. The Chief tries to put Van Lutyens' idea to him, but Robson is quick to note the influence of the interfering Dutchman and refuses. Robson goes straight for him, ready to tell him off for his interference, but Van Lutyens is ready. He insists there is something alive in the pipeline and the Chief backs him up. He heard something.

Robson is furious at this, certain that they are both imagining things and stirring up unnecessary trouble. However, the heartbeat sound starts up again, echoing clearly up from the impeller shaft. The Chief listens, as does Van Lutyens. Robson is still railing until the Dutchman shuts him up.

All ears are on the strange heartbeat sound issuing from the darkness of the impeller shaft. Everyone believes that there is something down there...everyone but Robson.

In the Harris' quarters, Mr. Harris continues to try and wake his wife, victim of a toxic gas attack. Jamie reaches out for the seaweed that Victoria spotted in the corner and the Doctor holds him back. He fears that it may be much more than an ordinary piece of seaweed, especially if it moves as Victoria said. Something like a spider. He produces a small bag and uses an implement to pick up the seaweed without touching it. Jamie holds the bag as the Doctor drops the green clump into it. He plans to analyse it as soon as he can.

Harris is distraught over his wife's comatose state. The Doctor is certain she'll be fine eventually, but he suggests getting her under proper medical care immediately. Harris rushes off to the staff medicare facility to do so, leaving the Doctor and his friends alone.

Realising that they are no longer under observation, the Doctor goes into action quickly. He wishes to get to the TARDIS so that he can analyse the seaweed sample.

Van Lutyens is certain that the heartbeat sound, audible to everyone, will be enough to convince Robson that there is something inside the pipeline, blocking the impeller. Unbelievably, the leader clings to the idea that there is a mechanical fault only. Not only that, but he orders his staff to have it fixed in half an hour. Then he storms out.

The Doctor, Jamie, and Victoria have had no trouble reaching the TARDIS and are hard at work on their specimen. The Doctor subjects a portion of it to natural gas to test its reaction. Victoria is doing a Bunsen test which results in some pronounced iron deposits. The Doctor deduces that the weed is giving out some kind of toxic gas.

Jamie observes a piece of the weed under a microscope and becomes alarmed when he sees "little wriggly things" in it. The Doctor is also alarmed and joins him at the microscope. It is "molecular movement" and the Doctor concludes, deadly serious, that this piece of weed is as much alive as any human.

Harris returns to the control area and bumps into Van Lutyens. Van Lutyens tries to get him to address the problem with Robson but Harris is in no mood. He is almost hysterical in trying to get help for his wife. He learns that Dr. Patterson has not returned from Rig D and there has been no communication in some time. At the very least he'll have Maggie taken to the medicare unit to be under observation by the matron there. Price hops to it.

Robson addresses Harris, sensing weakness in his concern for his wife. He snidely suggests that she might have a hangover! He doesn't accept the Doctor as a medical expert and distrusts any diagnosis he might have made. However, Robson becomes concerned when he learns that the Doctor and his friends were left alone in the residential block. Harris have left the prisoners unguarded!

Harris doesn't care about this one bit. His only concern is his wife. She's in a coma and the apartment was full of toxic gas when he arrived. She's been poisoned. A full-scale row erupts between the two men, Robson concerned about the escape of the "saboteurs" and Harris only concerned about the apparent attack on his wife. Robson implies that the Doctor and his friends could be responsible for it, as well as all the problems with the pressure in the pipe line. Of course, they'll never know now that they've been allowed to go free.

Their argument is interrupted by the Chief Engineer, who reports that the impeller is moving again. A triumphal Robson fells vindicated in his diagnosis of a mechanical fault, ignoring the fact that it just started up again on its own. The Chief and his men have done nothing to get it going again. However, the Chief does warn him that they can't keep it going. As if in response, it slows and stops once again, infuriating Robson.

Van Lutyens tries to tell him that shutting off the gas flow and unjamming the base of the impeller is the only way to solve the problem but Robson refuses to listen. He will not have a 30-year reputation ruined. Van Lutyens tries to tell him that there is more at stake than his reputation. Whatever has gotten into the pipe line is a menace and a threat to them all. Robson becomes apoplectic and even his loyal staff begin to worry about him. All eyes are on him and he feels it keenly.

Robson orders the impeller repaired and working fully in half an hour. No excuses. He heads off to his quarters, a wild look in his eyes.

Van Lutyens stands by, stunned. He is certain that Robson is beginning to crack up under the pressure and he turns to Harris for help. The poor man is so worried about his wife that he cannot think straight, but Van Lutyens convinces him to stay here. Maggie will be well looked-after in the medicare unit. Harris is needed here in this emergency. Reluctantly, he decides to stay.

Harris calls the Chief over, Robson's long-time friend, and asks him for his opinion. The Chief tries to downplay the latest incident but does admit that Robson is under pressure. It might be getting to him. Van Lutyens is certain that's the case and that he is much farther gone than he has even shown. For all his protestations of knowing all about life on the rig and gas business, Robson does not have experience of emergency situations. He is ignoring his advisers and even the evidence of his own eyes and ears. This is a crisis of command.

Van Lutyens' suggestion is to close the entire compound and evacuate the rigs. That is the only way to ensure the safety of the workers and the operation. The Chief knows that Robson will not agree to this, despite all the evidence. Two rigs with communications problems, a jammed impeller, pressure losses, and something trapped in the pipe line. They must do something, and Robson must be convinced to act. The Chief agrees and decides to help Van Lutyens and Harris talk to him.

In the TARDIS, the Doctor has produced a dusty old book and is flipping pages. He finds a listing with a picture Victoria recognises - the weed creature she saw in the oxygen store room. The book is one of legends and superstitions and the drawing came from by mariners in north sea in the middle of the 18th century. Somehow, this creature is real, and it has some connection to normal seaweed.

Suddenly, Jamie erupts into a sneeze. It is the toxic gas from the weed. They all turn back to the workbench to see one of the weed specimens has grown alarmingly and is nearly bursting out of its tank. It is now filled with foam as well and the heartbeat sound fills the room.

The Doctor and Jamie struggle to get the lid back on the tank to trap it inside but they have great difficulty. The weed actually fights them! Victoria is horrified at the sight of the mobile weed and screams loudly. Suddenly the weed goes limp and slides back into the tank. Jamie and the Doctor complete their task and replace the lid. It is just as the Doctor suspected - the weed formations are feeding off the natural gas under the sea and giving off a toxic version as they grow.

The Doctor is in a hurry to return to the Harris' quarters but does stop to wonder why the weed relented so suddenly. Very curious indeed.

In the control centre, Robson reaches new decibels in anger, having heard and dismissed everything his advisers have told him. He shouts and rants, embracing the paranoid delusion that they are all out to ruin him and push him from command. He is especially angry that Van Lutyens and Harris have corrupted his friend, the Chief. His rantings become more and more irrational and he repeats that he is in command. All of this is his responsibility! Seeing the looks in the eyes of his advisers, he races from the room, his mind snapped.

Van Lutyens is convinced that Robson is not fit to command and decides to get on to his people in the Hague. He must do something to get Robson replaced and to sort out the emergency before lives are lost.

Robson retreats to his quarters and slams the door behind him. His mind is reeling and he cannot concentrate. He is disturbed when someone outside tries to open his cabin door. He shouts at them to go away.

However, the visitor is the sinister Mr. Oak. He operates the ventilation system controls just outside the door, smiling evilly as he does so.

Inside, the vents open wide and the heartbeat sound issues from them. Robson looks up just as a mass of foam and seaweed comes pouring from the vent, hitting him square in the face. He cries out in pain.

Harris, on his way to the cabin, hears the noise and bursts in. Robson is screaming and staggering, panic in his voice. He clutches at his face as he pushes past Harris and races off down the corridor. Harris is stunned, not sure what to make of this extraordinary event.

The Doctor, Jamie, and Victoria have returned to the compound and made their way to the Harris' quarters. No one is about, but when Jamie sneezes, they know that there is gas building up inside. The Doctor and Victoria go to the bedroom to search for the source. They are shocked to find the floor of the cabin completely covered in foam and seaweed. The heartbeat sound issues loudly and the foam surges toward them. They retreat quickly, hoping that Mrs. Harris was gotten out before the seaweed got in.

They hear a shout from Jamie, who has gone to investigate the kitchen, and they rush to him. He, too, has found a mass of seaweed and foam and it now surrounds him. He stands atop a small table with nowhere to go. The door to the terrace is wide open and more of the stuff pours in.

The Doctor, thinking quickly, spies some curtains over a window and tears them down. He tosses one end to Jamie and has him tie it around one of the table legs. He and Victoria grab hold of the other end and begin pulling. They heave on the makeshift rope, trying to drag Jamie and his table out of the sea of foam.

Harris returns to Robson's quarters with Van Lutyens. There is no foam or weed to be seen, but the ventilator grille is broken. Harris insists he saw the foam and weed and that Robson was attacked by it. Amazingly, Van Lutyens believes this and it ties in with what happened to Victoria in the oxygen store. There is some kind of weed creature inside the pipeline and it comes out when it needs to - to suit its purpose - and then retreats back where it came from. The question is: what is its purpose?

They also must try and find Robson. Harris is certain he's out of his mind now and he could do himself some harm. Security must be alerted. Van Lutyens is more concerned about the seaweed, though, and is certain that it can emerge from any of the emergency ventilators whenever and wherever it likes. Harris agrees and decides they must order all of the grilles kept shut.

Van Lutyens insists that Harris take official command of the compound, taking over from Robson, but he balks at this. Robson is still officially in charge but Harris does issue the necessary orders. Robson will be found and the grilles will be kept shut. There is just one more thing he must do - inform the Director of Euro Sea Gas of the situation. Harris steels himself and decides to do so. He orders Price to get one Megan Jones on the line.

Jamie has been safely extricated from the seaweed and he and his friends head to the control area. Jamie is somewhat the worse for wear as they walk the corridors. Victoria seems concerned about all the trouble that they keep landing up in and admits to the Doctor that she's not sure she likes being scared out of her wits every second. This is worrisome to him but she doesn't elaborate. They continue on their way, but even Jamie notices something wrong with Victoria.

Megan Jones is contacted and will be arriving in three hours time. Harris is a bit nervous, hoping that she will understand why he did what he did. Van Lutyens assures him that he will stand by Harris in dealing with the board of directors. However, there is still the question of these creatures in the pipeline.

The Doctor arrives and overhears the last part of the conversation. He thinks he can shed some light on this part of the problem. He says that the creatures are composed of seaweed, seaweed that is "dangerously alive". Somehow, these seaweed formations have "life like human beings". They have invaded the coastline and are in the pipeline as well. Van Lutyens accepts all of this and wishes to take some action, but the Doctor assures him it is dangerous. The seaweed gives off toxic gas to protect itself and it is parasitic - it can attach itself to other living things. This thought chills Harris, whose wife was stung by a piece of the stuff.

Harris orders Price to check on his wife in the medicare unit whilst he tells the Doctor of the attack on Robson. He ran off in a wild state and no one has seen him since. Harris is convinced he saw one of the weed creatures in the room.

Price reports that the matron has not yet had time to go and fetch Maggie from her quarters but will do so now. The Doctor is stunned. They've just come from the Harris' quarters and there was no sign of Maggie. And the place was covered in foam and weed. Harris is shocked at the implication. What can have happened to his wife?! Van Lutyens turns to the Doctor, but he is helpless. He has no idea where she can be.

Maggie Harris is in truth on the beach outside the compound, gazing blankly out to sea. The water is covered with the evil foam. She is soon joined by another person - Robson. Both of them move and speak like possessed beings, slow and meticulous. Maggie tells Robson that there is little time. He understands what he must do.

In the control compound, efforts continue to re-establish communications with Rig D. Nothing. What is worse, now there is no answer from Rig A either. That's three rigs now out of communications. Van Lutyens is very worried indeed, and not just about those rigs not answering. The others will know of the blackout and the men will become concerned. That stress will do no good at all. The Doctor suggests sending someone to investigate but Van Lutyens says he has no authority to order it. Only Harris can do that and he is not here. They can do nothing but wait for the time being.

In the impeller area, the heartbeat sound starts up again. The Chief Engineer and his men are deeply worried.

Jamie and Victoria are having a rest in one of the cabins. Victoria is very worried about the situation and still quite agitated. Jamie tells her not to worry, the Doctor will think of something. He always does. She doesn't really doubt that, but she wonders what they'll do next. Where will they go? What new horror will they encounter?

Jamie looks at her quizzically. The unpredictability of their adventures is what makes it fun for him. He thought that she felt the same way but he is now beginning to wonder. She has been acting very strangely since they arrived here. Victoria tries to downplay her fears, sure that Jamie can't understand. Jamie closes his eyes and dozes off. But as Victoria considers the mystery of the weed creature, she becomes more and more frightened.

She is startled when the Doctor enters the cabin suddenly and she jumps. Jamie doesn't even stir. The Doctor seems to have come to speak only to her and his voice is gentle. He tries to calm her but she knows that something is going on. The heartbeat noise has been heard again and there's nothing that can be done about it right now. This seems to unnerve Victoria even more. The longer they wait, the more likely those creatures are to come and attack them. The Doctor tries to tell her it is not as dire as all that, but he really doesn't have a leg to stand on. Even he doesn't know how bad it really is.

Victoria repeats her assertion that everywhere they go, they end up in trouble. There's always something horrible - Daleks or Cybermen or Yeti. Why can't they ever go someplace pleasant? Someplace where there is no fighting. Just peace and happiness.

Harris, still distraught over the disappearance of his wife, has made his way to the beach in his search. To his astonishment, he finds Robson standing there, staring out to sea. He asks if Robson has seen Maggie and the response is cryptic: "You will find her. Very soon." Without another word, Robson walks away from him along the beach, gazing straight ahead as if in a trance. Harris shouts after him, stunned at the bizarre behaviour.

Van Lutyens has decided to take some action in Harris' absence and go down into the impeller shaft to investigate where he is certain the blockage has occurred. The Chief Engineer agrees with the Doctor that this is a very dangerous venture but Van Lutyens accepts the risk. He is going alone so that no one else will be endangered. It is all he can do in his unofficial capacity and he feels he must, despite the danger.

Van Lutyens is lowered down into the impeller shaft in a lift basket, being operated by a seemingly anonymous maintenance operative. It is Mr. Oak. The descent is relatively short and is soon engulfed in total darkness. He has a torch, but it does little to shed light in the aggressive gloom.

As he looks about, he is unaware of movement all around him. Suddenly he is attacked by a weed creature and he cries out. The Chief Engineer orders the basket raised immediately and everyone crowds around the impeller shaft to try and see what is happening down below. To their horror, the basket returns empty!

The Chief races off to find Harris whilst the Doctor looks curiously at the basket. He knows what he must do - go down after Van Lutyens. Victoria is horrified and begs him not to, but he is determined. Jamie balks at first, but cannot allow the Doctor to go alone. He too will go down.

Harris returns to the control area, looking haggard and weary. He can hardly believe what is going on and is distraught over the loss of his wife. Price reports that Megan Jones is on her way here right now. He also reports no news on Maggie's whereabouts. Harris seems to care very little for anything else, but heads off to the impeller area.

He is horrified to discover that the Doctor and Jamie have gone down the impeller shaft after Van Lutyens. None of them should have attempted such a thing in the first place. He orders Oak to raise the lift basket immediately to at least get the Doctor and Jamie out, but it is too late. It is already at the bottom and they are out of the basket.

In the darkness of the impeller shaft, Jamie and the Doctor move very slowly. Jamie finds it particularly creepy down there. They find Van Lutyens' torch abandoned on the ground but no sign of him. The Doctor is keen to continue searching, but Jamie is ready to leave. He feels as if there's something evil lurking down here with them.

Megan Jones, Board Chair of Euro Sea Gas and a no-nonsense woman used to being in charge, arrives at the control compound with her aide, the officious Perkins. They can't see any sign of trouble, although Megan doesn't expect to find any. She knows Robson is an efficient leader and should have everything under control, despite the vague warnings from Harris. Perkins finds this odd, as Harris said he was "taking over". Nothing vague about that. However, Megan knows that Robson is not a man that one just pushes aside, especially a young and inexperienced man like Harris. She sent Harris hoping that he would gain from Robson's experience. Clearly that hasn't worked. She's sure it's just an internal squabble between the two of them and she heads off to find out.

Harris, Victoria, and the Chief stand helplessly over the impeller. There is nothing they can do to bring the Doctor and Jamie back until they return to the basket. Price reports that Ms. Jones has arrived and Harris goes to meet her, bringing the Chief along for support.

Victoria is left alone with Oak and Quill, who both man the lift controls. Oak assures her that they'll take care of everything. Somehow, she is not reassured by his tone.

Megan, of course, does not believe a word of Harris' story about seaweed creatures, despite his insistence that he saw them with his own eyes. She has heard about the "accident" his wife has suffered and she thinks that he is distraught and imagining things. Harris continues to insist.

Megan can focus only on one fact - this complex is producing no gas and the country's supplies are already suffering. Harris reminds her that they are losing contact with their rigs one by one and she seizes on this as a practical situation which they can investigate. Harris suggests calling out air defence, which Megan thinks is a total overreaction. Perhaps Harris has gone paranoid. Company helicopters will do just fine.

Megan is now ready to speak with Robson, but Harris tries to dissuade her. He says that Robson is "not very well" and that "something" has happened to him, but can't be any more precise. She becomes angry at this hedging and wants to know exactly what happened. How can he possibly explain?

The Doctor and Jamie can see very little in the gloom and Jamie still suggests they leave. The Doctor leads them just a bit further down the tunnel...until a weed creature comes into view ahead of them. It's now time to leave. In a hurry!

The Doctor and Jamie race back to the lift basket and signal for the operators above to raise it. To their horror, nothing happens. Unbeknownst to them, the impeller area is now empty. Oak, Quill, and Victoria are gone.

The Doctor and Jamie shouts upward to try and get some attention, all the while keeping an eye on the approaching creature.

Oak and Quill emerge from a side room silently and without Victoria. They leave the impeller area, ignoring the shouts from below.

Luckily, the Doctor spots the service ladder on the inside of the impeller shaft and he and Jamie use that to get out of the shaft. They do so just as the creature reaches the lift basket. It seems to fill the entire shaft below them.

Jamie emerges first and helps the Doctor out. They are quick to notice the silence and emptiness. No one is here. Jamie knows that Victoria would desert them so something must be wrong. The Doctor agrees and the two of them split up to try and find her.

Megan Jones is stunned to learn that Harris was right. Their helicopter pilot has flown over all of the silent rigs and discovered foam and seaweed infesting them all. There are no other signs of life. She is at a loss as to what to do. However, Harris is not. The lives of the men on the other rigs is of paramount importance. They must all be evacuated immediately and the rigs - all of them - must be blown up to destroy this "evil from the sea".

Perkins is horrified by this suggestion, certain Harris is overreacting. Megan silences his protests but agrees with the point. Even if they wanted to, the minister would never agree to such a drastic plan. Suddenly their conversation is interrupted as Robson bursts in, wild-eyed and raving. He says that the rigs are against him, trying to get at him. He built them with his own flesh and blood, now they want to destroy him. It seems as if someone else's voice is coming from his lips.

For a second, Robson calms and seems to regain himself, but he seems lost and dazed. He says he's all right, but it is clear he's not. He stumbles off out of the room and Harris lets him go. He thinks that the strain of events has affected Robson's mind, but he is the least of their worries. However, the Doctor arrives and tells him he's wrong.

Megan Jones doesn't like the look of this scruffy newcomer but the Doctor ignores her for the moment. He tells Harris of their adventure in the impeller shaft. There is no sign of Van Lutyens and there is weed and foam all through the shaft. The Doctor also says that Robson is being controlled by some force that emanates from the weed. Megan is quick to dismiss this, but Harris reminds her that the Doctor's been right about everything so far. He thinks it's time they started to believe him. Reluctantly, Megan agrees to listen.

Meanwhile, Jamie searches the corridors of the control complex for Victoria.

The Doctor continues his explanation of what is happening, saying that the seaweed is intelligent, parasitic, and capable of exercising mental control over the beings it takes over. The Doctor calls it a battle of matter over mind. Robson, Maggie Harris, and even Van Lutyens have been taken over. And there are probably many more out on those rigs. They have become "part of the colony".

Price reports a communication from the Control Rig. It is Baxter again. The report is horrifying. He says that the seaweed is all around them and they cannot escape. He begs for help, for someone to get them out of here. His cries increase in intensity and then the screen goes blank. Communications are lost once again.

Jamie has been unsuccessful in his search and he returns to the impeller area. As he passes, he notices a door ajar along the corridor. He decides to take a look. To his surprise, Victoria is inside, unconscious on the floor. He wakes her up, having been more concerned than usual for her safety. She is touched, but chides him for his concern.

Victoria explains that the culprits were the "two engineers" who were with them in the impeller area, although she doesn't know their names. She also cannot clearly remember what happened to her. Victoria becomes angry at the thought of this, returning to her earlier complaint that they end up in trouble wherever they go. She says she's fed up with moving from one crisis to the next and says she just wants some peace and quiet. Jamie fears that she's not happy with the Doctor and Jamie, but she says that's not it. However, before she can elaborate, her attention is drawn by the heartbeat sound which begins issuing from the impeller room.

They hurry out to find the source and see the transparent piping filled with seaweed and foam!

With the Control Rig now gone, the Doctor concludes that the weed is trying to take over all of the rigs and form one vast colony. Its objective: the saturation of the British Isles and perhaps the entire world. Megan is stunned, as is Perkins, but the Doctor assures them that it is all very possible unless they can find the nerve centre of the weed and destroy it. However, finding it will be the most difficult part.

Jamie and Victoria rush in to report the weed has invaded the pipes. They drag him and the others out to see.

Out in the impeller area, the others are all shocked by what they see. The Doctor is not. He is deadly calm when he tells them that this is the adVance guard. The first part of the invasion. His words chill them all. The Doctor moves nearer to the transparent pipe, his eyes fixed on the moving, flowing weed and foam inside.

In the midst of the chaos in the impeller area, Price reports that he is now unable to raise any of the rigs over the communications equipment. He is apologetic but there is nothing he can do. Megan orders him tersely to keep trying.

The Doctor assures her that is a waste of time. By now, all of the rigs will have been taken over by the weed colony. When Harris asks what has happened to the crews, the Doctor tells him "the worst". Harris repeats his assertion that they should blow up the rigs in order to destroy the weed, but the Doctor knows this is the worst possible suggestion. It would simply spread the menace and make it impossible to successfully combat. It seems there is no solution.

The Doctor tries to reason out the best way to attack by analysing the weed's patterns. He believes that this weed was initially drawn up by one of the drilling rigs. When an engineer went to clear what he thought was "debris" from the bore hole, he must have touched it and became the first to be controlled by it. From there, the weed has worked to a purpose - it has deliberately attacked top priority people in order to gain control of them.

As unbelievable as it sounds, Megan believes this story and realises that the weed now controls two people who know the layout and structure of the entire compound, rigs and all. Robson is still around somewhere. He must be found and confined, so that the weed cannot use him against them. Harris agrees and orders a security search of the whole compound.

Harris turns back to the Doctor with a puzzle. If the weed is attacking high priority people in the compound, why did it go after Victoria in the oxygen store? She has nothing to do with the workings here. The Doctor has been puzzling over this as well. He believes that she accidentally disturbed someone who was interfering with the oxygen supplies. He and Harris conclude that pure oxygen must be toxic to the weed...and to the people it controls. If they're right, they can use oxygen as a weapon against the weed. Megan believes that they are right and that this is their only hope.

Unnoticed by anyone, two men overhear their conversation. Mr. Oak and Mr. Quill exchange a glance and then slip away without a word.

Oak and Quill head immediately to the oxygen store room and set about completing the task which Victoria interrupted. With gas masks on their faces, they open all of the oxygen cylinders in the room.

Price reports to Harris that Robson has been found. Oddly enough, he was just lying on his bunk in his cabin. A guard has been placed at the door to keep him in. Megan wants to see Robson. Harris thinks this a bad idea in his state, but she insists. They are old friends and she thinks somehow he might talk to her and reveal some useful information. Harris reluctantly agrees but decides to go with her. Perkins and the Chief Engineer come along as well.

Perkins tries to take his boss aside along the way, suggesting perhaps that military force is the best option. Megan rejects this, worried about the men still on the rigs. She can't order an assault that would kill them. Despite all that the Doctor has told her, she insists on believing that they are still alive and there may be some way to rescue them. She does at least believe that the Doctor may be right about an explosion spreading the menace.

All this does nothing to soothe Perkins' troubled brown. They hurry along to Robson's cabin.

When they enter, they find Robson looking like a shell of his blustery self. He is drawn and pale and his eyes hollow. Megan talks to him, trying to find some spark of recognition. She is encouraged when he says her name. She says she's here to help but he insists - his voice dead - that there is no help for him. No one can help. Megan knows this is not the John Robson she knows and she urges him to fight whatever is affecting him. She grabs and shakes him to try and get him to pull himself together, appealing to his ego and his pride.

Suddenly there is a true recognition. For a short moment, Robson regains himself and sees who is with him. He begs Megan to help him, pleading and pitiful. Then just as quickly he is gone again, sinking back down to his bunk, blank-eyed. Harris and Perkins urge her to leave him be, to let him go and let him rest. Sadly, she does so. She thought she had helped him regain rationality for a moment, but that moment is gone now.

Outside the cabin, Megan pulls herself together. There seems to be no hope for Robson but she is determined to put an end to this menace. They must destroy the weed. She marches off determinedly, Harris and Perkins in her wake.

Jamie and Victoria watch the Doctor as he wrestles with the problem before them. He looks particularly worried and his concern is in turn worrying Jamie. Victoria wishes they could just pile into the TARDIS and leave this all behind, but she knows the Doctor wouldn't do that. She becomes irritated when she realises that the situation could well be hopeless. The Doctor has said there's an obvious answer but he certainly hasn't come up with it yet. She still harbours a great deal of concern over the constant danger they find themselves in.

Megan and the others burst in, looking for some answers from the Doctor. They must have a way of attacking the weed. Unfortunately, the Doctor has no idea. Megan asks what he thinks the weed will do next and he says it will most likely attack them here in the control compound. This is the centre of gas distribution and it must have control over that. The only way of stopping the weed is to find and destroy its nerve centre before it attacks them. And they're not even sure how to attack the nerve centre if and when they find it.

Perkins suggests using the oxygen but the Doctor thinks this a long shot. Megan orders the oxygen stores put under guard so that they at least have that option.

In his cabin, Robson is awakened by the growing heartbeat sound. His eyes focus on nothing and he appears to be hearing instructions in his mind. He accepts them and heads for the door. The guard outside is taken completely by surprise. Robson clubs him down and then races off.

Harris learns of the total loss of the oxygen stores. That avenue of defense is lost to them. Clearly there is someone else in the compound who is working for the weed. It could be anyone and finding them will be very difficult indeed. Harris suggests consulting the employee files. There are fingerprint checks in there that may be useful. The Doctor goes with Harris to investigate.

Meanwhile, Victoria notices Oak and Quill quietly leaving the area. She remembers them from the impeller area and she goes to follow them. Jamie joins her, shouting for the two men to stop. They increase their speed and Jamie attacks. There is a brief struggle during which Jamie is almost gassed by the two men, but he recovers and continues to fight. Victoria screams just as Jamie lashes out at Quill. He folds like a house of cards. Mr. Oak, however, gets away.

The Doctor and Harris join them. Jamie is very pleased at his fighting prowess, but the Doctor has a suspicion that it wasn't Jamie's fist that knocked out Quill. Before he can explain, the Chief Engineer calls them all into the pipe room.

Megan stops Harris on the way, wanting to know what the flap is. He doesn't know and hurries on. Perkins is astounded by all of this and wonders what on Earth they can do to stop this menace. Megan is at a loss, preferring to trust the Doctor. He may be odd, but he does seem to be able to understand and anticipate what is happening. Perkins finds that rather odd in itself.

Inside the transparent pipe, the weed appears to be expanding, thrashing even more violently. The heartbeat sound is incredibly loud, seeming to emanate from everywhere. The Doctor is very concerned. This is what the weed did in the TARDIS and he should have foreseen that it would happen here too. While all eyes are on the pipe, a hand reaches out and clamps itself around Victoria's mouth. She is dragged away. However, this is no ordinary human hand - it is a hand with seaweed growing out of it!

Price arrives to warn everyone that Robson has escaped from his cabin, but that is the least of their immediate problems. Jamie sees it first and calls everyone's attention to it - the transparent pipe is starting to burst. The Doctor is horrified. The weed is expanding even faster than he thought. He urges everyone out of the room. When they are out, Jamie and the Doctor suddenly realise that Victoria is not with them.

They start their search immediately, hoping she can't have got far. However, they find some of the doors along the corridor closed against them. It is as if someone is directing their movements. They bang on the doors with no response. Moving along, they find a path open to them and they take it. However, more of the shutter doors are beginning to close and they soon will be trapped. The Doctor throws himself in the path of one of them and holds it, but it is too powerful. He calls out to Jamie for help.

They are lucky enough to make it through unharmed, but the door slides shut behind them. They cannot go back. They decide to split up to search for Victoria, both of them certain that the deck has been stacked against them.

Victoria's kidnapper is none other than Robson himself! He is halfway to becoming a seaweed creature himself, with tendrils of weed growing off of his skin. He has taken the unconscious Victoria out of the compound all together. He puts her into the passenger seat of a small car and zooms off.

Chaos has broken out inside the control room. Harris orders the pipe room sealed off to try and prevent the weed from breaking free. Alarm sirens sound all over the compound. An urgent message is sent to all guards. They must find Robson immediately!

Robson makes for a helipad and commandeers a helicopter, dumping Victoria inside. He takes off, the weed using his knowledge of flying for its own ends.

The control room is at last alerted to Robson's actions and Price relays the information to a stunned Harris.

The Doctor and Jamie return to the control room after their unsuccessful search. Each hopes the other has found her. They are disappointed and Harris cannot help at first. However, as he receives more information on Robson's escape, he learns that Victoria is with him in the helicopter. The Doctor is very worried and asks to be connected to the helicopter. Price switches on the R/T.

The Doctor calls out to Robson, trying to reach his human mind, hoping it hasn't been too corrupted by the influence of the weed. He urges him to resist the weed's mental control. But it doesn't work. Robson tells him that Victoria is his hostage. If he wants her to live, he will have to "come over to us". Then he switches off.

Megan Jones and Mr. Harris are confused by this remark but the Doctor is not. Clearly the weed creature sees the Doctor as a threat and it must have him under its control if it is to succeed. The Doctor is more than willing to go to the creature, much to Megan's consternation. He is needed here and she refuses to let him go.

However, Harris trusts the Doctor and offers to help. It is their only chance. He and Jamie will take another helicopter and follow Robson. The Doctor is certain that Robson will lead them to the nerve centre of the weed. As Megan fumes, Harris leads the Doctor and Jamie to some transport to the helipad.

Robson continues his flight.

Shortly the Doctor and Jamie are airborne, being flown by a pilot on a pursuit course. The Doctor calls in and says that he cannot make visual contact with Robson's helicopter. Harris receives information from Price and learns that Robson has already landed - on the Control Rig. The Doctor receives the information gratefully and the pilot changes course.

Megan feels helpless and hopeless. There is clearly nothing they can do while they wait for the Doctor, but that is against her nature and she feels restless. Harris can only sympathise. However, he tells her that he can only give the Doctor an hour. If he has not succeeded in that time, Harris will evacuate the control compound! Megan is outraged at this usurpation of her authority but a grim Harris will brook no argument.

The Doctor's helicopter approaches the Control Rig in short order. Jamie points out that one of the towers is completely covered in white foam. As the Doctor suspected, this is the control centre. Robson has led them right to it. His helicopter is visible on the landing pad. Jamie is worried when he discovers that they are planning to land there too - right in the midst of the enemy.

Shortly, the helicopter touches down and Jamie and the Doctor emerge. The landing area is covered in foam and is slippery. Jamie is reluctant to venture forth but the Doctor urges him on. They bid goodbye to their pilot, who takes the chopper away as soon as his passengers are clear. He is to hover above the rig just in case the Doctor and Jamie need rescuing.

Warily, the two men venture into the depths of the rig. It is dark and silent and Jamie can hardly believe that Victoria is here. He starts to call out but the Doctor stops him in time. He doesn't want to give away their presence just yet. However, he has been heard. Victoria calls out to them both.

Jamie starts to move toward the voice but the Doctor holds him back, afraid it may be a trap. They move forward cautiously and as quietly as they can. They check each doorway along the corridor but find nothing. That is, until they find a closed door from which emanates the sinister sound of the heartbeat.

Steeling themselves, they open the door and enter. What they see makes them gasp. Robson is there, looking crazed and nearly converted into a weed creature. A sea of foam surrounds him, as do tendrils of weed. He looks as if he is in the centre of the beast.

The Doctor challenges Robson and is told that "our new masters" have need of him. From another room, Victoria calls again. Jamie escapes to go and find her. Robson continues his ultimatum, uninterested in the boy. He tells the Doctor that he is going to assist in the conquest of Earth. The human mind and body do not exist any longer. They are tired and dead. Only "our new masters" can offer them life.

Whilst Jamie frees a grateful Victoria, the Doctor tries to override Robson's mental conditioning. He tells him desperately that his mind is being controlled and that he can resist. Robson opens his mouth and breathes toxic gas toward the Doctor, causing him to recoil. He covers his mouth and nose with a handkerchief and stumbles out of the room, calling out to Jamie.

Robson follows them out, repeating his whispered mantra that the Doctor must join them. Victoria sees the horrible apparition and screams. Robson immediately stops moving and recoils from the sound. The Doctor sees this and urges Victoria to scream again. When she does so, Robson recoils and clutches his head, begging for her to stop. The noise drives him back into the room.

The Doctor and his friends can now make good their escape. He doesn't bother even locking Robson in the room. There's no time. They hurry up the stairs to their waiting helicopter.

However, the pilot cannot see them in the mass of foam that now engulfs the top of the rig. They shout and wave but cannot make themselves seen. They must think of something else.

Jamie and the Doctor close the hatchway door in case Robson is following and they are safe for the moment, but they must still find a way to get off the rig. The Doctor spots Robson's helicopter sitting on the pad and races for it. Jamie is worried as they haven't got the pilot, but is even more worried when the Doctor gleefully volunteers.

With more luck than skill, the Doctor manages to get the chopper started. With a bit more fiddling of the controls, he gathers up enough steam to lift off. However, it is an awkward and dizzying ascent. Jamie and Victoria are becoming more concerned by the second. The Doctor thinks it "a very primitive machine" and thinks it should be easy to learn. However, several sickening aerial manoeuvres later they are no farther from the Control Rig than when they started.

To their rescue comes the pilot of the other helicopter who radios instructions to the Doctor. By following them, the Doctor is able to gain control of the craft and learns how to ascend and to move forward. Just one problem remains as they head back toward land: how to make it land!

Back in the control compound, an eerie silence reigns. Harris' nerves are on edge and he despairs of ever hearing from the Doctor. It has been nearly an hour and he is ready to order an evacuation of the compound for the sake of all the men under his command. Megan appeals to him to wait at least the full hour to hear from the Doctor. But Harris thinks they've already waited too long. The weed has burst through the transparent pipe and now fills the pipe room, growing every second. Soon it will burst free and they have nothing with which to fight it. He sees evacuation as their only option.

Megan tries to calm him, thinking of concrete action. She orders Perkins to get on to the Defence Minister and try to organise as much oxygen as possible. He does as she orders, but Harris fears it is too little too late. That weed will be on them in short order and they are defenceless.

All are stunned when the Doctor and his friends enter the control room, alive and well. They report finding Robson but not Maggie Harris. Mr. Harris is forlorn and angry and frightened. There is absolutely no way they can fight this menace. But the Doctor says he is wrong.

The Doctor reports that they stopped and the medicare unit on their way back here and discovered that the man that Jamie knocked out - Mr. Quill - has almost completely recovered from his possession by the weed. The growths on his arms have disappeared and he is now alert and seemingly in control of his faculties. All eyes are on the Doctor as he reveals that the weapon that saved the man was sound vibrations! To the astonishment of all, he says that it was Victoria's scream that killed the weed and freed Quill. Something about her particular pattern of sound that kills the weed.

Harris accepts this completely. It makes sense, given the whispering speech patterns of those affected by the weed. The noise must hurt them. The Doctor agrees and asks for one half hour's delay in the evacuation to make an effective attack against the weed using lots and lots of noise. With nothing else for it, Megan agrees to this. However, Harris despairs of ever stopping it. The weed is preparing to swamp the compound even as they speak and they must evacuate.

The Doctor is positive he can do it. They must attack the nerve centre of the weed, which is on the Control Rig. By using the pipeline itself as a conduit, they can transmit sound directly from the compound to the Control Rig. He tells Harris it is the only way to help his wife and all the others already in the grip of the weed. Harris agonises over the decision to remain, going so far as to check what is happening in the pipe room. The weed and foam are everywhere, threatening to burst out. But he decides to put his faith in the Doctor and hope they can keep the weed at bay for a half hour. He agrees to call off the evacuation.

Very pleased, the Doctor springs into action, quizzing Price on the equipment available. He finds transmitters and loud speakers all ready for his use, but the sound will have to be boosted to reach the Control Rig. They will also need a tape recorder to record Victoria's screams. He will make a loop of it so that it can be repeated. Price and the Doctor fall deep into discussion.

The Chief Engineer arrives and draws the others to the impeller area. From the impeller shaft, they can see a tide of foam and weed rising up. It will soon emerge to swamp the compound.

A bit later, the Doctor and Jamie are organising speakers which will be attached to the pipeline. Meanwhile, Price is trying to record Victoria screaming. However, she is unable to do so on cue. Price reports this to the Doctor who is surprised. In all their adventures, Victoria has been a champion screamer but she insists she can't just scream for no reason.

Suddenly, looking past them into the impeller area, Victoria begins screaming her lungs out. Harris, the Chief and many others race from the impeller area and seal the doors. The weed and foam have surged out and swamped the area. They are all trapped in the control room and the weed slams against the doors, trying to burst them open. Victoria continues to scream and scream whilst the Doctor, Jamie and Price recoil from the noise. The tape recorder is rolling and all is recorded. The Doctor is pleased.

Harris tries to get them away from the doorway but they continue with their plans. Harris, convinced the cause is lost, decides that he must evacuate. Megan begs him to stop but he is adamant. Only the Doctor's appeal stays him. Reluctantly, he agrees to let the Doctor continue.

Back on track, the Doctor reports that the scream is ready. He has created a multiple-speaker system through which the scream will be broadcast. He also indicates a gadget of his own hooked into the system that will, he says, create a sonic laser sound wave down the pipeline. He is certain that it will destroy the nerve centre of the weed. Unfortunately, it will not be effective on the weed here in the control complex. They have ordinary speakers to deal with it. They must simply hold it off as best they can until the boosted sound has done its work.

Whilst the Doctor finalises the set up, Price reports that more weed has invaded the compound. It now appears to be bursting from every inlet. Harris and the Chief struggle to close the corridor doors and seal it off, but they are too late. Several of the areas are swamped. Each new avenue leads right to the control room. The Doctor works desperately to complete the sound system as the foam and weed bears down all around them.

All doors are closed - the control room is completely hemmed in now - and the weed pushes against them. Soon it will burst through and they will all be dead. At last the Doctor prepares two speakers and gives one to Jamie. He moves everyone out of the way so that they will not be affected by the sound and orders Price to switch on. The foam is already starting to break down the door and Price watches it, transfixed. He cannot move.

The Doctor races to him, speaking urgently to snap him out of his trance. After a moment he does so and activates the controls. Victoria's amplified screams pour from the speakers at deafening volume, repeated over and over. To Megan's horror, nothing seems to be happening. The weed continues to force its way into the control room. They're done for!

However, after a moment, it becomes clear that the sound is having an effect. The forward movement slows and eventually stops. The foam and weed are no longer adVancing. In fact, its motion begins to reverse and the weed retreats. This pattern speeds up and soon it is gone from the control room. In fact, looking at the monitors they can see that the weed is retreating all along the corridor, returning to the outlet from which it came.

It worked!

At the Doctor's order, Price switches off the screams and silence returns. Everyone is quite stunned from the events and from the piercing sound. No one can quite believe what has happened. It worked here, but has it worked at the nerve centre?

Harris orders helicopters sent out to all the rigs to investigate and he and Price try to reestablish communications with the rigs. Meanwhile, Megan thanks the Doctor profusely for his help.

In one corner of the room, Victoria weeps uncontrollably. The Doctor goes to her worriedly and she collapses into his arms. She is in shock, still frightened and stunned. She can barely speak, devastated by recent events. The Doctor comforts her as best he can.

Harris reports that he has made contact with the rigs and discovered that everyone is safe. They are all free of the weed! The Doctor leads Victoria to the communications equipment where they can see Maggie Harris on the monitor. She is alive and well, although quite the worse for wear. Robson is there too, reporting that the weed has completely gone from the Control Rig. Harris is amazed to discover that Van Lutyens is alive and well also. Maggie assures him that they are all fine. It's over.

Harris tries to contain his joy and relief, keeping a professional tone. He tells them that a helicopter is on the way and that they'll be back home within the hour. Cheers rise up on the rig and in the compound.

Some time later, a celebration is being held in the Harris' quarters. The Harrises, Robson, the Doctor, Jamie, and Victoria have all enjoyed a fine meal and are sitting around reveling in the normality of their lives. Soon Robson is tired and wishes to retire. He says he'll see the Doctor in the morning and learns that he and is friends are ready to leave. Their destination, says the Doctor, is undecided. Robson accepts this and invites them back any time. However, they should try the front door next time, just to avoid any "unfortunate incidents".

Robson leaves and the Doctor stands to go. Jamie rises with him but Victoria does not. She seems glued to the spot, staring forward and remaining silent. The Doctor quickly realises what is happening: she doesn't want to go. She says she doesn't want to leave the Doctor and Jamie, but her desire to remain here is clear in her tone. The Doctor is very understanding but Jamie is shocked and angered. The Doctor tells her that if she wishes to stay here and settle down, then that is what she must do, despite what he and Jamie might want. Maggie Harris offers to take her in, delighted to have her for as long as she wants to stay. Victoria is well pleased and the two women embrace.

The Doctor, calm and benevolent, thanks the Harrises. He decides that he and Jamie will stay for another day, just in case Victoria decides to change her mind. Jamie bursts out angrily, hoping to change Victoria's mind, but the Doctor silences him again. It is her decision to make, and they must abide by it. Jamie's sadness is clear.

Before retiring for the night, Robson checks in on Price in the control room. Everything is back to normal and functioning perfectly. Relief crews are on their way. Price is weary but determined to stay on until relief arrives. Robson bids him good night and heads to the impeller area to see the Chief Engineer.

The Chief is still working over his equipment. The main control valve is still slightly damaged and he is overseeing the repairs. Should be sound as a bell by the morning. Robson orders him to rest up and leave the repairs to the relief crew. It can wait. The Chief agrees.

Robson barks a good-natured order at the rest of the crew, just to show who's still boss, and the Chief smiles. Nice to see Robson back to his old self again. The two old friends bid each other good night.

Before they turn in, Jamie and Victoria have a quiet talk about her decision. He still hopes that she's not determined to stay, but it is clear that she is. She has found a safe home environment here and is ready to settle down - to leave the constant adventuring and danger behind. However, that doesn't make it any easier for her to leave Jamie and the Doctor.

Jamie tries one more time, making sure she'll be happy here in this strange future. It is much different than her own time. She agrees, but knows that there is nothing for her back in her time. She has no home or family left there. The Harrises are nice people and she will be fine. Seeing that her mind is made up, Jamie decides to leave her be.

He starts to go but Victoria calls him back. She makes sure he won't slip away tomorrow without saying goodbye. He promises he won't. They both retire for the night.

The next morning, the three friends say their final goodbyes on a windswept beach. Then Jamie and the Doctor board their raft for the trip back to the bobbing TARDIS.

Once on board, the Doctor quickly activates the controls. The ship raises up out of the water and into the air. On the scanner, Jamie watches sadly as Victoria waves goodbye to them. Her image grows smaller and smaller as they rise.

Jamie is devastated at leaving her but the Doctor reminds him it was her decision to go. She'll be fine with the Harrises and Jamie shouldn't worry. Jamie is angry at this and sulks, refusing to even notice that it is time for them to go.

The Doctor gazes at Jamie in silence for a moment, his own emotions overflowing. He tells Jamie, a hint of anger in his own voice, that he was fond of her too! Then he activates the dematerialisation control.