Something gets onto a huge space station. It finds its way into a tube that is shaped like a man where inside, a man seems to be asleep.

The TARDIS appears on the Space Station Nerva with the Doctor calling Harry a clumsy ham fisted idiot. Harry just wanted to open the door and sent the TARDIS on this long journey in time. Sarah chides the Doctor that this was just supposed to be a trip to the moon to prove to Harry that the TARDIS does work. The Doctor claims Harry switched the Helmic Regulator quite far. Harry continues to babble about what a wonder the TARDIS and that the Doctor could make lots of money with it, selling it to bobbies, and in Trafalgar Square. Sarah tells him to do stop burbling but then when he tells her it must be shock, she relates, "That's how I felt the first time." The pair recognize a lack of oxygen while the Doctor also plays with a yo-yo. He tells Sarah it is a gravity check. The Doctor wants to get the power on -- the lights are off. When Sarah reckons on wandering, Harry tells her he wants to stick with the Doctor, who gets the lights on. At a wall panel, Harry touches a switch which closes a door slide over the small control room that Sarah has wandered into.

The Doctor tells Harry that the station devices are probably late 29th or early 30th century to Harry's amazement. The Doctor finds a magro slave drive and a Bennet Oscillator.

Sarah finds the air even less in the room she is in and bangs on the door.

The Doctor tells Harry since he gave that helmic regulator quite a twist, the TARDIS must have taken them far beyond the 30th century. Finally the two realize Sarah is gone. The Doctor says, "I've told her time and again about going off by herself." The Doctor notices it is a vacum tight door and remarks that the station is "an early space vessel." Harry recalls he touched a switch and then has to find out which one. When he does, they get the door open and rush in to a passed out Sarah but fall prey to the same trap: the door closes, now trapping all three of them. Harry tells the Doctor he always hated sliding doors ever since he caught his nose in one in the Pompey barracks. The air supply is low; the Doctor finds not much is working in this room, and Harry comments, "It's all my fault." Working, the Doctor claims the fault, "I got us into this, Harry." The Doctor uses the sonic screwdriver to find shared cables and the oxygen valve servo mechanism. Something sabotaged the air supply -- a something that bit through the cables with large teeth, "There's a mystery here, Harry." The Doctor gets them air and opens the door. He and Harry lay Sarah on a couch.

Harry calms a reviving Sarah, "Steady on, old girl." Sarah opens her eyes, "Harry, call me old girl again and I'll spit in your eye." The Doctor calls to her, "Welcome back, Sarah Jane." Harry wonders if a bit of brandy will do her any good. Sarah tells him he knows she hates brandy. Knowing they have some in the TARDIS, Harry and the Doctor move to the other, larger room where the TARDIS is but a laser device fires at them. It burns one of Harry's shoes as the Doctor pushes him out of the way of the laser. The two take refuge behind a desk.

On the couch, Sarah is put to sleep and vanishes from the room.

The auto guard laser device burns the Doctor's hat as he tests it. This is one of the things that was cut out--when the Doctor restored power, he unknowingly reactivated it.

Sarah has appeared on another couch in another room, a couch set in the wall. A male voice welcomes her as a sister, welcoming her to space station Nerva. She is on a tranquil couch. The voice of the High Minister, a female voice, talks to Sarah.

The Doctor uses the sonic screwdriver to unscrew the desk he and Harry are hiding behind and they move it to the far wall. The laser only detects and fires on organic items. They inch the desk to the auto guard cut off switch on the far wall. The Doctor tries to use his scarf but it burns it a bit. Harry asks for a cricket ball -- with one he would be able to hit the lever down. The Doctor produces one from his pocket but the ball is lasered to bits when Harry throws it. He asks for Harry's other shoe and with it, makes Harry divert the laser, hoping it is not double barrelled. The Doctor, while the laser is busy with Harry's other shoe, shuts the lever down. He complains about Madam Nostradamus having made his scarf, one of a kind, irreplaceable.

Gas starts to irradiate Sarah, putting her into a deeper sleep. She cannot fight it.

The Doctor finds Sarah gone and calls himself a fool. The couch is a short range matter transfer unit, internal and cannot have sent Sarah far. He and Harry go out into a hallway where large windows show outer space. They pass by an armoury, go to Area Q, and a sterile area. Harry, in the hallway, sees something moving which makes the Doctor scoff...that is until he finds green slime left by something. A trail of glop. The pair go into a decontamination chamber and a light comes on. Finally, they go into a room, a larger room with banks on a wall. Animal and botanic specimens are there. Harry is not feeling better. The Doctor tells him to pull himself together. He shows Harry cryogenic chambers -- everything that was worth preserving is here. On microfilm records they find files of music, history, artecture, engineering, and other sciences. All from Earth.

Harry mentions this station is like a survival kit. To which the Doctor states, "You're improving Harry, your mind is beginning to work." When Harry looks proud, the Doctor adds, "It's entirely due to my influence -- you mustn't take any credit." More seriously, he asks, "What's happened to the human species, Harry?"

The two find many larger rooms with high ceilings and large amounts of body sized and shaped tubes with what Harry thinks are dead bodies... hundreds, perhaps thousands of bodies. A mortuary, Harry infers. The Doctor goes to another room, "Homo Sapiens. What an inventive, invincible species. It's only a few million years since they crawled up out of the mud and learned to walk. Puny, defenseless bipeds. They've survived floods, famine and plague. They've survived cosmic wars and holocausts, and now here they are, out among the stars, waiting to begin a new life. Ready to outsit eternity. They're indomitable."

The bug thing falls to the floor at Harry's feet. It has been dead for some time. Harry finds the emergency med kit. A head medical person, a woman, Vira, starts to revive and they bring her the kit. She finishes reviving herself, prompting Harry to say, "Independent sort of bird, isn't she?" When she fully comes out of it, she is odd, cold hearted at first. They need the help of a doctor of this time, the Doctor telling her that his doctorate is purely honorary. They show her Sarah who has undergone what Vira calls tissue irradiation. Sarah hasn't been that way for more than an hour -- Harry telling Vira that they haven't been here for more than an hour. There seems to be a small communication problem as the speech that Harry and the Doctor use, especially Harry, are strange to Vira, who is First Medtech. The process works on an electrical field which draws power from the biosphere. Vira goes to the leader, Lazar, called Noah, a name from mythology Vira claims as an amusement. The Doctor surprises Vira by telling her several things: the Earth is not dead, the solar flares she believed destroyed life happened longer than 5000 years ago -- she and her people on Nerva have overslept by several thousand years. The systems failed and something else got in -- they show her the creature. By the size of its brain pan, the Doctor feels it is intelligent and was here for a purpose. It sort of stopped their alarm clock.

A green blob of a worm like structure is in the engines -- a solar stack. The worm turns a lever with a tentacle that reaches out like a psuedo pod.

As she attempts to revive Noah, Vira is interrupted by a technical fault in the main power supply, caused by the worm creature, unknown to any of the humans and the Doctor. The Doctor has noticed there is a secondary power supply. Harry tells Vira, "Gremlins can get into everything, old girl. It's the first law of the sea."

The Doctor goes to a power room and gets the power going again, ready to go to the solar stacks where the power fault came from. He calls via radio and tells this to Harry and Vira.

Vira revives Noah and they hold hands. Noah calls Harry a regressive. Vira believes they are travellers in space as they claimed to be. Noah thinks that they might be spies from Colony 9 but Vira reminds him of that colony's zero -- zero survival rate -- there was only one generation. Noah insists that aliens and strangers threaten Nerva's mission. They must be eliminated. Vira wants the Council to decide that and reminds him of this. Noah goes after the Doctor while Vira helps revive Sarah more fully and as she does, Harry puts his hand on her shoulder. She looks at it.

Noah stops by the armoury to get a stun gun.

Vira tells Harry, "Leave now, Noah will not permit contamination." Harry argues that if it weren't for he and the Doctor they would still be asleep. Sarah revives as Vira puts forth her position, "Personally, I do not think your destruction is necessary." Harry says, "Thank you very much," sarcastically. He gets Sarah up and around.

The Doctor finds goo of green in the solar stacks and one of the psuedo pods looks as if it has an eye on it.

Harry shows Sarah the creature's body, she is repulsed and hopes it is dead for sure. Vira goes to Palate 3 to revive Power Systems Technician Dune... and finds his body gone. She asks what Harry and Sarah have done with him.

In the main control room, Noah stops the Doctor from shutting down the power in the main solar stack switch. Noah says, "The Earth is ours." When the Doctor tries to make nice with him and talk, Noah shoots him down with the stun gun.

In the solar stack engine room, the worm thing breaks out of the glass.

Noah calls Vira, prompting Harry to ask Sarah of Vira and Noah, "Cocky bunch, aren't they?" Sarah and Harry leave Vira to go find the Doctor when they hear that Noah did something to him. Noah tells Vira via radio that he is going to the solar stack to find out what sabotage the Doctor was up to. When Noah goes there, the giant green worm thing rises up and hits his hand, leaving slime on Noah's hand. Noah passes out.

Harry, in the control room, listens for the Doctor's heart beats and hears them both beating fine. The Doctor suddenly rises up quickly, realizing he was cut off in mid sentence, "I think I was saying something important....I WAS saying something important..." He hates stun guns. When Sarah and Harry tell him what Noah is doing, the Doctor calls Noah an idiot. He gets up and starts out. When Sarah and Harry fear for the Doctor's condition, he says, "Never mind about me, Harry, there's a man in danger!"

En route, the Doctor explains that the reason Vira wouldn't stop them is that is not her function. By this future time, mankind and womankind are highly specialized. It was not her job to stop them. She is a med tech and cares for the medical issues only. Noah stops all three of them in the hallway, telling them the observation port is damaged. The Doctor is about to suggest they return to the cryo rooms when Noah insists they go that way anyway... the Doctor at least tells Noah he was thinking of going back there anyway. All this time, Noah keeps his hand hidden.

In cryo, Vira revives Libri who reacts badly when he sees Noah, "Keep back!" The Doctor asks Libri what he saw but Noah keeps him and his two companions silent, threatening them with his stun gun to their faces. Noah leaves, claiming he is doing so to shut down the systems. His order to Libri is to kill the three time space travellers if they try to leave. Before he leaves, he acts even stranger -- telling Vira to stop reviving more humans and he claims, "I'm Dune." He leaves, telling Vira, "No more aliens." This is a reference to the Doctor, Sarah and Harry -- two of whom are human!

Vira thinks the power surge and loss during Noah's revival is making him act strange. The Doctor reminds her of Libri's subconscious impression of something horrible...when he looked at Noah. Vira continues to debate the Doctor, who comments, "You take some convincing that we're on your side." He finds the membrane of an egg shell in Dune's body compartment, "It's too horrible to think about." The thing that is dead on the floor has its egg tube empty -- it is a queen colonizer like a type of wasp on Earth, it lays its eggs in bodies. On Earth the wasps lay eggs in caterpillars so that the larvae have a fresh food supply as soon as they hatch, "Strange how the same life patterns are found reoccurring throughout the universe." When Vira asks of Dune and his knowledge, the Doctor says, "Thoroughly digested, I'm afraid." When Sarah chides him, grimly, "Don't make jokes like that, Doctor," the Doctor reiterates his very real fears, "Sarah, when I say I'm afraid, I'm not making jokes."

The High Minister's Voice delivers what Harry Sullivan calls a pep talk to those she expected to revive. She tells them they have been entrusted with a sacred duty to protect human culture, human trust, human faith, and love so that these shall never perish. They have slept for longer than all of previously recorded human history. Harry comments to Sarah about the High Minister being female and adds, "Fancy a member of the fair sex being top of the totem pole." Noah calls Vira and tells her to intensify the safety checks, warning them they are in great danger. He tells her she must get the humans to Earth before the Wirrn... he gives command to Vira. Noah is taken over again and tells them, "We shall absorb the humans -- the Earth shall be ours." Noah fights it and his human hand fights the Wirrn hand. He tells them Libri is dead. Noah tells them the Wirrn are in his mind and he goes into the room the TARDIS is in. The Doctor hears all of this over the intercom and thinks the Wirrn may have some sort of endo parasitism. He talks to himself and Sarah tells Vira why, "He talks to himself sometimes because he's the only one who understands what he's saying." The Doctor comes out of it and he needs Vira to go with him, telling her that her duties have been widened now that she is in command. Harry can handle the revivification -- one gram of strophium and the injection over the pectoral major. As The Doctor and Vira leave, Sarah moves to follow but she hears the Doctor call back, "And Sarah, you stay and help Harry." Harry turns to her, "Yes, come along nurse Smith."

Vira and the Doctor find Noah -- his left side of his face and his left shoulder are covered in green glop and he drops the gun he was holding. Noah falls. Vira tells the Doctor that she and Noah were pair bonded for the new life.

Sarah talks to two newly revived technicians, Rogin and Lycett, "I'm sorry. It must be a terrible shock." Rogin complains that he knew there would be a snitch up and he said this 5000 years ago. Lycett corrects him, "10,000 years ago." Rogin wishes they had stayed in the shelters when the flares came, they would have been happily dead by now. The two men think that Dune died from lung tissue atrophying. Sarah shows them the body of the dead Queen, "Something got in here."

The Doctor and Vira arrive. The Doctor wants to examine the Queen so Harry pulls it over. The Doctor finds the throaxtic incision and tells Harry to cut there. If they can find a weakness in it before the larva hatch. The Doctor moves to Vira to implore her not to revive the other humans as she plans. She was also planning to override the safety precautions to do so. It would take four days to revive and evacuate all the suspended humans on Nerva. The Doctor tells her she cannot do it -- in hours the creatures will be moving and hatched. He levels logic to her: if she does it and they fail, her people will die in pain and fear but if he fails, only the six of them awake now will know of it. In between the larva and adult stage there must be a pupa stage where they are dormant and cannot move against them. He realizes the larva in the infrastructure in the solar stacks needed solar radiation. Rogin complains that he liked living on the Earth. The Doctor reckons the creatures' lungs can recycle wastes -- CO2 and O2, meaning the things can live in space, only visiting planets occasionally for food. The Doctor calls them marvellous. Noah spoke of a great blackness. He had the race memory of a Wirrn, something called symbiotic activism. Using extension leads and a neural cortex amplifier, the Doctor will hook the skin of the Wirrn to the video circuits. Harry asks what the Doctor is doing exactly. He tells Harry: there are latent epidermal impressions left in the skin. Gypsies used to say the eye retains the last image before death and this is not wrong in a way. When it doesn't work, he decides to hook it through to his cerebral cortex using this as a core stimulus.

In the connecting room, a green larva listens at the vent.

Vira objects to the Doctor's plan since it means great danger to him -- it could burn out his living brain. The Doctor smiles at her, "An ordinary brain perhaps but mine is exceptional." When Sarah objects as well, the Doctor tells her it not just their lives at stake but the whole of humanity, "It may be irrational of me but human beings are quite my favourite species." The Doctor hands a gun to Vira and tells her to use it if she had to and orders Lycett to connect him -- he has to order the man to do it twice and yells at him to. Vira tells Sarah that if something goes wrong or if the connection is broken before it finishes, the Doctor might remain a part of the Wirrn mind forever.

Rogin and Lycett hear the vent grill fall out in the next room and go to investigate.

On the video hook up, Vira, Sarah, and Harry see the Wirrn Queen arrive, get shot from the auto laser guard, and make its way to Dune -- where it laid its eggs -- so the larva could eat him and digest his knowledge!

The larva thing corners and kills Lycett. Rogin moves into the room and pushes Harry to safety as he closes the door to prevent it from getting in. Harry moves to take the Doctor off the connection before it is finished but Sarah stops him--it would kill the Doctor. Harry and Rogin leave to get more guns. In the hallway, after they get guns, Noah chases them out of the armoury, shooting at them. Rogin shoots Noah -- whose entire top is covered by the green infection.

Vira shoots Noah; Sarah is with her, screaming, "Run, Doctor, run!!!" The Doctor does so and joins the two women on the steps out of the room. Noah asks Vira to stay, then to abandon the ark. The creature tells them that the humans came to the "old lands" and destroyed the Wirrn breeding colonies, forcing the Wirrn out into space; they fought humans for a thousand years. Star pioneers of humans succeeded in Andromeda, where the Wirrn are originally from and drove them out into space. The Wirrn live in space, the Doctor says, they don't need the ark. The Noah Wirrn tells him that he is a fool and knows nothing: the Wirrn breed on terrestrial ground. The Doctor tells it there is plenty of room in the galaxy for us all, to leave the ark. It tells him they used to use cattle as hosts but now they will use the humans in the cryo chambers. In one generation, using the knowledge of the humans there, they will become an advanced technological species. They used Dune's knowledge of physics and quantum mechanics. Vira is told if she stays she will die with the others. She is the last to leave the room.

Harry and Rogin wait. Rogin wants lights on -- he wants to know where he is putting his feet. Harry, still without shoes, tells him, "You should worry." They tense as someone comes. It is the Doctor, followed by Sarah Jane and Vira. The Doctor asks, "Anyone for a jelly baby?" Harry refers to the TARDIS, a "sort of spaceship thing in there." The Doctor tells him they cannot leave since Vira will not leave. Sarah tries to explain what just happened while the others think about what to do. The Doctor keeps stopping her from telling them what she wants to. Finally, she gets her word in. The plan: electrify the switch boxes to an electric defense so that the Wirrn cannot touch the cut out switches. They will need human bait; Rogin suggests the transport ship -- it has its own power and has twice the power of the ark. 100 meters away is a small conduit just small enough for someone the size of Sarah Jane to get through and bring the wire to the cryo chamber from the transport ship area. The resulting defense screen would protect the cryo chamber from the Wirrn. The Doctor says, "Good girl, Sarah."

Sarah, Rogin, Harry and Vira run to the transport ship, find the conduit, and drag a wire in. Rogin lowers the transport ship ladder platform and he, Vira, and Harry go up into it. Using a two way radio to guide Sarah, Rogin talks her through the maze of ducts to the correct one. In the ship, Rogin tells Vira they could head straight back home to Earth, "Nothing could stop us."

The Doctor is working in the control room to electrify the switches.

Harry calls Sarah, "How're you doing, old girl?" Sarah calls him a twit.

The Doctor hides from a Wirrn -- Noah by hiding in one of the empty cryo chamber beds. Noah-Wirrn leaves.

Rogin warns Sarah to move quietly over the next area -- it is the infrastructure which is full of Wirrn by now. Noah tries to get her through the vent but she moves past his section without getting touched by him.

From the cryo chamber, The Doctor opens a triangular vent.

Harry tells a struggling, panicking Sarah, "Stick at it." Sarah pushes herself, "That the trouble, I keep getting stuck!" Harry tells Rogin and Vira, "Good old Sarah -- leave it her to always keep up her sense of humour." Sarah rolls her eyes.

The Doctor shines a flashlight into the vent and Sarah sees it. She is close to the exit. But she gets stuck and starts to panic all over again. The Doctor goads her into getting mad enough to push her way through, yelling at her, telling her that they never should have relied on her, "Never!" He calls her a whimpering, whining girl, useless. Sarah says, "Oh Doctor!" He shouts back, "Oh Doctor, is that all you can say!" He calls her stupid, foolish girl and tells her she has no guts, thinks she is tough but isn't when the chips are down, "100s of lives at stake and all you can do is lie there blubbering!" Sarah gets so mad, she moves through the vent to the Doctor, "Just you wait till I...."

Sarah exits and when the Doctor starts to help her out, she hits him, "I can manage! I don't need your help, thank you very much!" The Doctor helps her out anyway, "Yes you do, yes you do." He pulls her out and she hits him away from her. He tells her he is very proud of her. She realizes she's been conned again and calls him a brute. Sarah moves to the access chamber to close the door and sees a Wirrn. It moves at her so she closes the door before it can get in. A blast hits Noah-Wirrn who is with two more Wirrn!

The Doctor reports to the others, "Don't let the power drop." The Doctor listens, "Either discretion is the better part of valour or..." Sarah asks him, "Or...?" "They're planning something." From the vents, an arm of the Wirrn pulls on Sarah's legs and she falls. It starts to pull her into the vent but the Doctor pulls her off of it but falls himself. He is starting to be pulled into the vent himself but he blasts it with the electric cable. It leaves. Sarah shivers, "Those things are so venomous! They never give up!" The Doctor adds, "Nor shall we. What we're protecting is too precious." They raise the power.

Noah calls Vira, "Your resistance is useless." He will offer them safe passage from the ark. The Doctor answers for Vira, telling Noah that Vira is busy resuscitating more humans. The Doctor appeals to Noah, "Leave us in peace." Noah tells him that the Wirrn can live for years without oxygen -- he will shut down the oxygen pumps. If the six humans agree, they will be allowed to leave. The Doctor tells him, "The Wirrn hate all humans. We'd be attacked as soon as we walked out the door." As swarm leader, Noah can make sure his swarm does not attack. Hearing this, the Doctor appeals to Noah's human part, telling him to lead the swarm into space -- that is where the Wirrn belong. He asks Noah to remember the Earth -- the wind, the sun, the fields, the blue sky -- he tells Noah that is for the human race. Noah hesitates but tells the Doctor that he has no memory of the Earth.

Harry doesn't want to be an alarmist but he thinks he saw something on one of the monitors aboard the transport ship. The Wirrn are trying to come up the funnel entrance ladder. Vira orders the starboard Four Negative Thrust Out fired. These blasts draw the Wirrn off. Vira starts to say something, "I wonder if Noah..."

Sarah tells the Doctor it might be her imagination but it feels stuffy in the cryo chamber. The Doctor immediately tells her it is her imagination. She gives him a smiling look, "You'd say that anyway."

The Wirrn space walk around the side of the ark and start to come into the starboard cargo hold -- they report to the Doctor that it looks like the entire swarm. They will get into the reversion vents and the interior bulkheads there are only low stress -- which means they will get into the ship. The Doctor instructs Rogin to cut the power, initiate automatic take off, and evacuate the ship of Harry, Vira, and himself. The Doctor prompts Sarah, after the electric power is off, to run through the halls. He follows her.

Harry watches the swarm on the screen but Rogin tells him to get out of the transport ship. Harry goes down. Rogin hesitates a moment before following, thinking about something. The Doctor and Sarah arrive; the Doctor tells Harry to go into the ark with the girls. Harry asks what he's going to do. The Doctor yells, "Just look after Sarah!" Harry follows the girls into the ark.

The Doctor begins releasing the synestic locks and puts one in his jacket pocket. Rogin tells him, "I thought that was your idea." But whoever releases the last lock will be killed by the blast. The Doctor tells him to get into the ark with the others, "There's no sense in our both being killed." Rogin hits the Doctor in the face, knocking him out, telling him that the space technicians union demarcation disputes would never allow the Doctor to do a job that a space tech should be doing. Rogin releases the last one and he is blasted down as the rocket takes off.

Vira, Harry and Sarah watch as the rocket lifts off. Vira figures Rogin and the Doctor both must have died instantly. Sarah walks off a bit and nods NO. Harry goes to her, "C'mon Sarah, he'd have wanted you to be brave." The Doctor appears at the doorway and tells them he woke up in a protection hatch -- Rogin's bravery saved his life and their own as well...and there may have been something more...perhaps a small vestige of human spirit left in Noah, "Was Noah on their side and one step ahead of them in the end."

This proves true as Noah calls them, "Goodbye Vira." Noah deliberately neglected to set the rocket's stabilizers -- causing the rocket to blow up with all the swarm of Wirrn on board. He must have known that would happen. The Doctor stares, "More than a vestige of humanity."

Vira walks off, her turn to be upset, her hands on her face. The Doctor moves her, telling her it is time to begin now. It can all begin again. With the transport ship gone, she will have to rely on the matter transmitter to get her people back to Earth. It can only send three at a time. The Doctor finds it not functioning properly -- the signal is faulty, nothing serious he deduces, maybe just a spot of corrosion on the Earth end. He plans to just beam down and fix it. He takes the TARDIS key from around his neck and has Sarah fetch his coat and hat from the TARDIS. Sarah and Harry return with coats also, Sarah in a garish yellow rainjacket and wool hat.