Somewhere in England, two men are descending a metal ladder into a cave - "potholing" on their time off. They reach the cave floor and began to cautiously explore their surroundings. They hear a strange distant roar but shrug it off for the moment. One of them walks ahead and takes a turn around a rock wall. Suddenly, the roar sounds again -- louder. He looks up to find a huge reptilian creature towering over him, its massive jaws open as it closes in. He screams as it attacks him, knocking him to the ground. His companion flees for his very life.

Meanwhile, the Doctor is happily at work on his newest acquisition. All that can be seen are his long legs sticking out from under a little yellow roadster -- soon the be known as Bessie. A license plate marked WHO1 is at his feet. He is singing happily as he tinkers with the Edwardian sports car.

But he is interrupted by the arrival his assistant Liz Shaw. She's skeptical that the car will ever work, but the Doctor is cheerfully optimistic, eager to take it for his first drive. But before he can go joy-riding, she reads out a message. Brigadier Lethbridge-Stewart has asked to report "forthwith" to Wenley Moor. At first, the Doctor refuses to obey the orders, but Liz tells him the area is famous for a huge system of caves. They could go exploring. Intrigued, the Doctor agrees to go. He starts up the car (to Liz's astonishment) and the two head out, his roadster speeding through London and into the countryside.

In Wenley Moor, an intense middle-aged man (Dr. Lawrence) is addressing his staff. Brigadier Lethbridge-Stewart sits to one side in his UNIT uniform. Lawrence tells them to remain calm and assures them they will solve their problems -- despite the presence of UNIT. During his speech, a soldier enters and summons the Brigadier. He leaves and finds that the Doctor and Liz have just arrived. He tells them they are in an atomic research center deep in a cave. Before they can ask any more questions, he drags them into the conference room where Lawrence is concluding his meeting.

Lethbridge-Stewart quickly introduces everyone, including the very friendly Dr. Quinn and the rather suspicious and reserved Major Baker. He asks Lawrence to brief them on the installation. Lawrence explains that they are using a cyclotron to bombard atoms with subatomic particles. The goal is to create a source of safe cheap atomic energy by converting nuclear energy directly to electricity.

But the Brigadier notes that there have been two major problems: personnel problems involving nervous breakdowns and, of greater concern, unexplained power leakages from the reactor. He now wants a three-pronged attack on these problems -- with Baker working on security, Liz investigating the personnel problems and the Doctor looking into the reactor problems.

Quinn agrees to show the Doctor and Liz around. Lawrence tells the Brigadier to keep him updated but insists they not disturb the work. It is clear there is some tension between Lawrence and the Brigadier. He is also highly skeptical the Doctor can help.

After everyone leaves, Baker asks why UNIT has been called in. Lethbridge-Stewart replies that these are unusual events -- precisely what UNIT investigates. Baker believes the problems are caused by saboteurs working from the inside.

The Doctor, Liz and Quinn arrive in the control room. Quinn notes they are at the very center of the cyclotron. The Doctor worries that if a power leak happened at the wrong time, the reactor could turn into a bomb but Quinn tells him that Lawrence is unwilling to slow down the research. While they are talking, Liz seems a little disoriented and the Doctor asks if she's all right. Liz insists she's fine and wants to get started on the personnel files.

Quinn and the Doctor talk about the caves. Quinn likes to explore them but is reluctant since an accident involving two technicians. One was killed and the other is still in the hospital. The institute seems to be keeping it quiet.

The Doctor asks if there is a pattern in the power losses. Quinn says there isn't but the Doctor wants to check for himself. He examines the log and finds that pages are missing. Quinn summons Miss Dawson and asks her about it. She tells him that Spencer -- the injured technician -- was responsible for the log and that explains the discrepancy. But the Doctor notices that the pages were ripped out.

Liz meets with Dr. Meredith, the physician in charge. He tells her that they've had a lot of problems with breakdowns on their staff. He blames it on being underground for so long. The Doctor arrives and asks to see Spencer. At first Meredith refuses, but the Doctor is insistent. Meredith relents but warns the Doctor that it's not safe.

In one of the patient rooms, Spencer is crouched on the ground, drawing on the walls like cave man. Meredith tells them that Spencer's been drawing since he woke up -- with the exception of attacking Meredith. The Doctor approaches Spencer slowly and asks about the drawings -- particularly one of a strange-looking man with three eyes. The technician whirls and attacks him, grunting incoherently and trying to strangle him. The Doctor manages to calm Spencer down and the man resumes his drawing. The Doctor thinks that some deep terror has thrown the man's mind back millions of years.

In the control room, the technicians are testing the cyclotron. Quinn sends his assistant Roberts out of the room and Miss Dawson hurries over. The two have a frantic whispered conversation. Dawson asks Quinn to "tell them to stop, at least while these people are here." She also asks him to tell the Brigadier or Baker what's happening before someone else gets killed. But Quinn in unphased, telling her the others wouldn't understand or wouldn't believe him. He insists that the knowledge he stands to gain is worth any risk.

Baker meets again with the Brigadier. He's completed security checks on everyone there, but can't find any records on the Doctor. The Brigadier deflects him, saying that the Doctor is his personal responsibility. As if on cue, the Doctor arrives. After Baker leaves, the Brigadier tells the Doctor that Baker had a bad slip-up on his previous job and ended up at the institute. The Doctor tells the Brig that he's discovered enough already to get very worried.

In the control room, the power up continues. Lawrence pops in for moment, insisting that they not delay. Technician Roberts is losing his concentration and has to be reminded of the task at hand.

The Brigadier and the Doctor argue over his discoveries. The Brigadier thinks the cave drawing are unimportant but is concerned about the log. He tells them Doctor to keep investigating. The Doctor petulantly replies that the Brig has hardly been Sherlock Holmes. Suddenly, the lights dim. Once again, power is leaking from the cyclotron. "Come on, Watson!" says the Brigadier and they rush off to the control room.

Lawrence is steaming that the cyclotron is acting up again while Quinn is calmly shutting down the reactor in stages. Soon after the Brigadier and the Doctor arrive, Roberts begins shouting and refusing to shut down the reactor. The cyclotron starts to get out of control. Dawson rushes in to shut it down and Roberts savagely attacks her. The Doctor steps in and shuts down the reactor and while the Brigadier restrains Roberts.

Some time later, the Doctor and Liz are in an office. Liz has finished her look over the personnel files. She's concerned about a huge rate of neuroses -- almost all from people working in the cyclotron room. She remembers that she felt strange - almost terrified -- when she was there. The Doctor notes that it is the deepest room in the institute and closet to the caves. Everything keeps pointing back to the caves.

Liz then remembers that the post-mortem on the dead technician has been finished. He died from a blow to the head, but his body was raked with huge claw marks. The Doctor is astonished and now determined to explore the caves for himself.

The Doctor is exploring a cave in Wenley Moor, investigating the mysterious events at a top research center. A roar sounds out and he rounds a corner to find a huge dinosaur towering over him. The Doctor covers his face as the awesome reptile closes in for the kill.

But before it can attack, a high-pitched fluting signal rings out through the cave. The dinosaur hesitates and then moves away. The Doctor sits up and cautiously follows it a little ways, stopping to examine an enormous footprint it left on the ground.

The Brigadier and Liz are standing beside a map of the caves, trying to determine where the Doctor may have gone. The Brig is angry that Liz didn't tell him the Doctor was headed to the caves, but Liz says the Doctor didn't want anyone to know. The Brigadier and Baker are quickly organizing a team to search for the missing Time Lord. But before they can get going, the Doctor walks in, perfectly all right.

Drs. Lawrence and Quinn are in the control room, arguing about the cyclotron problems. Quinn and Dawson assure Lawrence that the cylcotron is mechanically in perfect working order, which Lawrence can't believe. He laments that they started out so well but is somewhat mollified that at least UNIT is having problems with the Doctor missing. After he leaves, Dawson tells Quinn she's worried that UNIT might still search the caves.

The Doctor has described his experience to everyone but they are skeptical. Liz tries to discuss it, asking the Doctor if it was prehistoric and he says it was not like any prehistoric creature he has seen. Baker suspiciously asks if he was referring to creatures he saw in museums and the Doctor backtracks and says he was. Baker returns to the idea of saboteurs -- perhaps they are faking this monster to keep people away. The Doctor concedes this is possible and the Brigadier decides they should investigate further. Liz is annoyed that they will not let her come along. The Doctor is concerned that the Brigadier and Baker seem keen on a violent solution to the problem.

The Doctor takes them to the cave but the giant footprint is gone. Before they can investigate further, they hear a loud roar. Baker, Captain Hawkins and two soldiers run to investigate. Baker hurries ahead and sees a man-like shape in the darkness. He opens fire, wounding it. But before he can give chase, a roar sounds out and he turns to find the allosaur towering over him. It strikes him down but the strange fluting tone summons it away before it can do further damage.

The Brigadier and everyone else arrive. Baker is stunned and confused, but alive. Hawkins explores further ahead. The Doctor finds Baker's gun - bent by the attack - and reconstructs the events. Hawkins calls them ahead and shows them ricochet marks and a trail of blood. The Brigadier gives chase but the Doctor stays behind to investigate the blood. He reminds the Brigadier that he doesn't really know what he's going after.

On the surface, a strange figure emerges into the setting sun. It stumbles around in confusion. It vision is strange, seeming to come from three eyes, one of them with a red haze over it.

The Doctor and Liz examine the blood in a microscope. Liz is surprised to find that it looks like reptile blood. The Brigadier returns -- they've lost the creature in the darkness but he will look again in the morning with more men. He's worried that Baker may have shot some innocent pot-holer. Liz tells him that Baker is hurt, but basically all right and the Doctor laments that Baker blazed away so eagerly. He reminds the Brigadier that the large dinosaur was called off before he or Baker could be killed. He theorizes that they are dealing with two creatures: a large ferocious one and small intelligent one that is not neccesssarily hostile.

The creature stumbles through the darkness, its breathing labored. It approaches a farm and a reptilian hand swims into its vision, opening a gate and a barn door. It stumbles into the barn, collapsing into the shelter of the hay.

Lawrence, meanwhile, is angrily confronting Miss Dawson in the control room. It seems that Quinn is nowhere to be found. The Doctor arrives, also looking for Quinn. Dawson tells them that he's just gone home, exhausted from working so hard. But Lawrence is still angry, sarcastically wondering if Quinn has gone to his private cottage to work on his book. He leaves in a huff. The Doctor is more pleasant but still determined to see Quinn about the power losses.

Quinn is not in his cottage, however. He is in the caves, following a map, watched in the distance by a small reptilian creature. A red light shines down on him, guides him through a secret door, down a hall and into a small room with a chair and a computer. A strange voice asks why he's come. He warns them about the Brigadier's search party. They reply that his warning is too late. One of them was attacked and wounded, fleeing to the surface. They are unconcerned about whether any humans were hurt. He asks them to stop stealing power from the reactor but they insists they still need it. He tells them that it was the power leakages that brought the soldiers and they ask him to help with the wounded one. At first, Quinn refuses, worried that he will be seen by the hunting soldiers. This angers the voice and it says that they will give him the knowledge he wants if he helps them. They promise to give him a summoning device and he sees a small metal object lying next to him. After he picks it up, the red light guides him out again.

It is morning on the farm. The farmer -- Squire -- head to the barn and begins to pitchfork the hay. Suddenly he sees a reptile arm flailing in the straw. He runs to the door and tells his wife Doris to call the police. The creatures rises up and approaches him, his image tripled in its vision. It knocks him to the ground with one blow. His wife runs in, sees the creature and screams.

The Brigadier and Hawkins are organizing their search for the creature when Lawrence bursts in. He's irritated that a military operation is being run out of his research center. He reminds them that they were imposed on him to solve his particular problems and asks them what progress they've made. The Brigadier tells him about Liz's findings of unusual amounts of stress in the staff, but Lawrence dismisses it and wants to know about the power losses. The Brigadier tells him the Doctor is checking it out. Lawrence doesn't believe the Doctor can find anything with his sonic screwdriver.

The Doctor arrives and informs Lawrence that the reactor is in perfect working order, making Lawrence even angrier. The Doctor suspects that some outside influence is affecting it and recommends the center be shut down until they figure out what's happening. This infuriates Lawrence and he says he will ask the undersecretary to recall them. Lawrence storms out and the Doctor call him a pompous idiot. In fine sarcastic form, he then ask the Brigadier if he's ready to start playing soldier again. Before Lethbridge-Stewart can answer, the phone rings. It is the police reporting a dead local farmer with claw marks on the body.

The Doctor goes with him to the farm. He examines the body and find claw marks on it, although they are smaller than the ones on the dead technician. The Brigadier asks him if still thinks the small creatures are not hostile and the Doctor points out the farmer attacked the creature with a pitchfork. Besides, the man did not die from the claws -- he died of fright!

They hurry off to see Doris in the hospital, leaving Liz to examine the scene. Doris is lying in bed, agitated and frightened. The Doctor gets a pen and paper and draw a man with three eyes -- similar to what Spencer was drawing on his wall. She reacts to the image with horror, telling him it killed her husband. Then she tells him it's still there in the barn! The Doctor and the Brigadier race off to save Liz.

Liz Shaw is carefully investigating the hay where a farm couple were attacked by a mysterious creature. Unseen by her, a reptilian shape emerges from the hayloft and descends the ladder. It approaches her, her shape tripled in its three eyes. She turns and screams as it knocks her to the ground. The creature then moves to the barn door, seeking to escape. But a policeman is outside. It quickly bolts the door and retreats into the darkness of the barn.

Dr. Quinn is at his cottage when Miss Dawson arrives. She tells him that Lawrence has been asking about him, but he dismisses it and excitedly shows her the summoning device -- the first product ever seen from another civilization. He tells her he hasn't seem them yet but they must trust him to ask for his help. Dawson isn't that impressed. She mentions that the soldiers are searching for something and investigating an attack at a local farm. Quinn quickly realizes there is cave opening near the farm and hurries out, again dismissing Dawson's concerns about Lawrence.

The Doctor and the Brigadier arrive at the barn, hurrying to rescue Liz from whatever menace still lurks there. They find the door bolted but the Brig quickly smashes it open. They find Liz unconscious on the floor. The soldiers search the barn while the Doctor tends to his assistant. Hawkins and the soldiers quickly return, reporting that the locked barn is empty.

Liz wakes up and tells the Doctor "I saw it" although she can only tell him that it walked upright. Then Hawkins finds a hole the creature broke through the side of the barn to get out. The Brigadier orders him to search the entire area.

Dr. Quinn arrives in the midst of this scene. He explains that he felt better after resting at his cottage and was on his way to the research center when he noticed the police cars. He asks Liz what happened but the Doctor cuts her off, looking at Quinn expectantly. Quinn prepares to head out but is stopped by the Brigadier who wants to know where someone could hide in the area.

Outside, the soldiers are searching the area thoroughly, firing flares as they clear areas and assisted by a helicopter.

Quinn is of little help to the Brigadier and prepares to leave again. The Doctor asks him if he could give Dr. Shaw a lift but the evasive Quinn claims he was actually on his way to the village because his car was acting up. After he leaves, Liz points out to the Doctor that the farm was miles out of Quinn's way.

Quinn stops along a deserted stretch of road, steps out and activates the signaling device. It gives off a sound exactly like the one used to summon away the dinosaur. Dogs bark frantically at it and the soldiers notice its eerie tone. A helicopter swoops over Quinn, who quickly hides the device, waves and gets back in his car.

The Doctor and the Brigadier roar up to Hawkins in Bessie. He mentions the sighting of Quinn and the funny noise - also having noticed that it was the same sound they heard in the cave. The Doctor and the Brigadier race off to investigate.

Quinn stops his car again. The signal is getting more and more frantic. The creature approaches him cautiously through the shrubs. At the last moment, Quinn turns to face it.

The Doctor and the Brigadier stop at once, noticing that the noise has stopped. The quickly drive to where it seemed to come from and find biped footprints and tire tracks.

Liz arrives at the center and visits Baker in the infirmary. Baker is eager to head out and get back in the investigation, but Liz insists the Brigadier wants him to stay in bed. She asks him what he remembers. He can only recall shooting at someone. He then asks her again what the Brigadier is doing about the situation and asks to see him. Liz placates him but after she leaves, he takes off his sling and hops out of bed.

Quinn is back at his cottage when the Doctor arrives. The scientist tries to get rid of the pesky Time Lord, but the latter keeps asking him pointless questions - why he's there, what his cottage is like -- purposefully not asking him about the missing creature. He especially notes how hot it is in the cottage. Quinn tells him the thermostat is broken. The Doctor then slyly comments that the cottage is like a reptile house and Quinn is clearly alarmed at this remark. He demands the Doctor leave. The Doctor does but not before asking Quinn if anything is wrong and telling him he could help. As a parting shot, he reminds him that they haven't caught the mysterious creature.

Later, the Doctor and Liz are searching Quinn's office, trying to find out more about him. The Doctor breaks open a cabinet and finds a globe in it. But the continental shapes are strange. They are bunched into one landmass -- the way the earth was 200 million years ago during the Silurian era.

Suddenly, Miss Dawson walks in, catching them in the act. She threatens to tell Lawrence but the Doctor confronts her, asking her what is going on in the caves. For a moment, she wavers, her concern for Quinn outweighing her loyalty. But before she can speak, the Brigadier comes in. Dawson clams up and hurries out.

Back in Quinn's dark cottage, Dawson arrives. She warns him the Doctor was searching his office and begs him to tell someone about what's going before more people get killed. She thinks he could trust the Doctor but Quinn thinks the Doctor would only steal his discoveries. Dawson replies that the creatures haven't given him a single discovery but Quinn is undeterred. He says that now that he has the creature, he can demand they tell him what he wants to know.

Back at the research center, there is staff meeting. The Doctor continues to insist that their troubles originate in the caves. Suddenly, Baker strides in. He insists that they should storm the caves to go after the saboteurs but the Doctor thinks this a terrible idea. The Brigadier makes Hawkins escort Baker back to sickbay and Lawrence is annoyed that the Brig has essentially arrested his security officer. The Doctor praises the Brigadier for being sensible, but the latter replies that he somewhat agrees with Baker and wants to search the caves with an armed force.

Lawrence stands up, clearly fed up with UNIT. He tells them that the undersecretary -- Mr. Masters -- is on his way and will almost certainly recall the Brigadier. He then heads out with Lethbridge-Stewart pursuing him, trying to change his mind.

The Doctor is frustrated with the entire situation. He decides to make one last attempt at pursuading Dr. Quinn to trust him.

It is now night. The Doctor arrives at the cottage and rings the bells with no answer. He opens the door and steps in, calling out to Quinn. As he moves into the living room, a reptilian hand appears in a doorway.

The Doctor has come to Dr. Quinn's cottage, eager for one last chance to pursuade the scientist to explain the mysterious events affecting the research center. But he is too late. Quinn is dead. The Doctor picks up the signal device still clutched in Quinn's hand and turns it on. Immediately, a man-shaped reptile enters. The Silurian revealed at last!

The Doctor doesn't panic. Instead he puts out his hand and introduces himself. The Silurian is timid and scared but doesn't attack. It indicates it can understand him. The Doctor starts asking it questions but a passing car startles the creature and it runs off, smashing through the cottage door.

Sgt. Baker is back in the infirmary, scribbling out plans on a piece of paper. He calls Sgt. Hart in to help him take off his jacket. The friendly soldier helps him but Baker attacks when the sergeant's back is turned. He then hurries away.

The Brigadier chews out the apologetic Hart for not being more cautious with Baker. Hart suggests a search but the Brigadier declines, sending Hart away. Liz asks about a search as well but the Brigadier points out that Baker will be headed to the caves to look for saboteurs. They'll find him once they organize their own expedition.

Baker is indeed in the caves, following his notes. He turns down a passage and steps into a puddle, which bubbles ferociously around his ankles, freezing him in place. Silurians swarm around him and he opens fire. But he runs out of ammunition before he can hit anything. A Silurian closes in and its third eye glows brightly. Baker falls to the ground under its gaze.

The Brigadier and Liz are waiting for more troops. The Brigadiers asks where the Doctor has got to and Liz tell him he went to visit Quinn. The Doctor arrives but doesn't tell the Brigadier what he found -- perhaps fearing it will provoke a military action. The Brigadiers asks him for suggestions and the Doctor again recommends shutting down the reactor. He also recommends a careful investigation of the caves, not a military action with soldiers crashing about. The Brigadier, losing patience with the situation, hurries out.

Liz asks the Doctor what's wrong and he tells her that Quinn was killed by a Silurian. He quickly describes his encounter with the creature. Liz is surprised it didn't attack him but the Doctor replies that the Silurians are clearly an alien life as intelligent as human beings and not hostile. Liz reminds him that it attacked her but the Doctor points out that it was trying to escape and acted as any human would.

The Doctor is more determined than ever to explore the caves. Liz insists on going with him this time, threatening to go the Brigadier if he doesn't bring her. The Doctor relents and tells her they will follow a map he found in Quinn's cottage.

They return to the caves. Liz wants to take a shortcut across the pool, but the Doctor finds Baker's spent rounds. He throws a rock in the pool and it boils up. They decide to stick to the map.

They finally come to the door Quinn used earlier. As they are puzzling out how to open it, Liz sees a Silurian approaching. They hide and watch as it opens the door with a summoning device. Liz touches the door and finds it hot -- an environment suited to reptiles. The Doctor then remembers the device he took from Quinn and uses it to open the door.

The two creep into the Silurians' lair, sneaking around the creatures. They find Baker in a cage, yelling defiantly at the Silurians. They sneak over and speak to him. When he finds out they've come without any troops, he tells them to hurry back and bring the Brigadier. He can't escape as the cage has an energy lock. The Silurians keeping asking him questions about humans -- their food, their weapons. He thinks they are going to invade. The Doctor advises him to cooperate a little in order to find out something about the creatures.

Then the lights dim. The Doctor and Liz look into another room. They see a large sarcophogus made of stone and glass. It is opening and a Silurian is slowing rising out of it. The Doctor guesses that they are raising him from some deep hibernation -- perhaps this explains the power losses.

Baker insists that they head out before they get caught. They sneak out by a different route and see the allosaur in one room. The Doctor hypothesizes that the Silurians are using it as a watchdog, although Liz reminds him that the dinosaurs have been extinct for millions of years.

Back at the research center, the personnel are calmly shutting down the reactor, which has had yet another mysterious power loss. Lawrence arrives and tries to get hold of Quinn or Dawson, but neither can be found. The phone rings and Lawrence is told that undersecretary Masters has arrived. Lawrence managed to straighten himself up before the undersecretary walks in.

They retire to the office. Masters tells Lawrence the Brigadier wants more troops. This infuriates Lawrence, who is angry the Brigadier wants to chase strange creatures instead of doing something about the power failures. Masters tells him the future of the center is in question. They've poured in millions of pounds with no results.

This provokes a rambling diatribe from Lawrence. He insists that the presence of others is not helping. Everyone has a crazy theory -- Baker thinks there are saboteurs in the caves while the Brigadier thinks there are monsters down there. Lawrence wants the UNIT people out so he can work on the power problems. He thinks the Brigadier is just trying to make himself look good by exaggerating the situation. He also disparages the Doctor, calling him a raving lunatic.

On cue, the Doctor and Liz arrive. The Doctor asks if there's been another power failure and when Lawrence confirms it, he turns to leave. The Brigadier arrives and asks Masters about reinforcements. But Masters tells him he can't ask for more troops to look for monsters in caves. The Brig decides to start his search anyway but the Doctor stops him, telling him he knows what's happening.

Meanwhile, the Silurians are interrogating Baker. He tells them that humans are the only intelligent life on the planet. When they ask about weapons, he refused to answer. They regard him like an animal, noting to each other that "it" is becoming aggressive again. The Young Silurian wants to force Baker to answer but the Old Silurian doesn't want him hurt. He laments that they killed Quinn since Baker is obviously less intelligent. He thinks they need to study the humans and learn about their civilization. But the Young Silurian thinks they should just wipe the humans out.

At the center, everyone is incredulous at the Doctor's story. The Brigadier starts asking how many Silurians there are and how they are armed, but the Doctor doesn't know. He wants Masters to go the Minister, but the undersecretary insists that he needs proof of such a fantastic story. The Brigadier still wants to go into the caves, although he emphasizes that doesn't want to start a war. The Doctor is adamant that the presence of armed men will inevitably lead to violence. Lawrence, reasonable for once, asks if there is any alternative. The Doctor says that he wants to go and talk to the Silurians.

At this crucial moment, a dazed Miss Dawson arrives. She tells them she went to Quinn's cottage and found that he had been killed by the Silurians. She admits they knew about the creatures. She tells them that Quinn found them while pot-holing and was cooperating to gain scientific knowledge from them. They repaid him by killing him. They Brigadier is now determined to go into caves, convinced they are hostile. He leads Miss Dawson out to get her statement but not before the Doctor asks if Quinn knew their intentions. She scoffs insisting that "we must destroy them before they destroy us."

As Masters and Lawerence confer, the Doctor decides to head into the caves and warn them about the soldiers. Perhaps they will listen to reason if the Brigadier won't.

The Silurians are working in their computer room when the Doctor suddenly appears and announces himself. They drag him into a cage, opening and closing it with energy from their third eyes. Baker wonders why the Doctor is there as do the Silurians. He warns them of the attack, tellng them that if they meet the soldiers in peace, there won't be a battle. They ask why he is doing this and the Doctor explains that he wants the humans and Silurians to live together in peace. This is the humans' planet now. The Silurians insist that this has always been their planet.

The Brigadier and Hawkins are following the map but find an unexpected wall. They turn back to find another wall has suddenly appeared behind them. They are trapped.

The Doctor is a prisoner of the Silurians, having come to warn them of a pending attack by the Brigadier. Baker is furious that he would do this, but the Doctor insists he was just trying to prevent a massacre. The Young Silurian arrives and the Doctor asks if they've met with the humans yet. He replies, "I have destroyed them. And now I will destroy you". His third eye glows and the Doctor grimaces in pain.

But before the Young Silurian can kill the Doctor, the Old Silurian arrives and stop him. He is angry, telling the Young Silurian he gave no orders to kill the humans. Now they will send more soldiers. He reminds the Young Silurian that there are millions of the creatures but the Young Silurian insists they can wipe them all out.

The UNIT troops are trying to dig through the walls enclosing them in the cave or radio back. But they can't do either. Hawkins and the Brigadier debate what to do and realize that they may only a few hours of air left.

Liz is in the research center, talking to Masters. Lawrence keeps dimissiing her arguments. He is convinced that the power losses are a technical problems and he just needs more money and people. The phone rings and Masters answers. The UNIT people are reporting they've lost contact with the Brigadier.

The Doctor tries to tell the Old Silurian that the humans are more dangerous than he thinks. Baker tries to violently shut him up but the Old Silurian uses his third eye to knock the soldier down. The Doctor then pursuades the Old Silurian to let him out of the cage.

The Brigadier has ordered digging stopped to conserve air. They notice that Private Roberts is shaking and hyperventilating, as though terrified.

Masters takes another call. He tells UNIT to continue the search. Miss Dawson arrives and argues that the Silurians have probably killed the soldiers. She tells Masters they should send more troops down, attack the Silurians. Lawrence again scoffs at the idea of cave monsters. Liz replies to Dawson, saying they shouldn't attack the Silurians. Suddenly, Masters asks where the Doctor has gotten to. Liz hesitates.

The Young Silurian and the Silurian Scientist are talking about the human weapons. The Young Silurian thinks they are primitive -- a fitting weapon for apes. But the Scientist thinks they have some ingenuity and reminds him one of them was wounded. The Young Silurian thinks the apes should be wiped out and is concerned their leader is talking to one.

Liz finally caves in and tells everyone the Doctor went into the caves to warn the Silurians. Dawson is appalled, believing the Brigadier and his men have probably been destroyed. She again presses to attack but Masters wants to wait for more information.

The Doctor tells the Old Silurian that the planet now belongs to man. The Old Silurians insists that it is their planet. He explains that a small planet was approaching Earth and they calculated it would draw off the atmosphere. So they built a shelter and went into hibernation until the atmosphere returned. The Doctor realizes the planet became the Earth's moon and the catastrophe never happened.

Baker starts yelling for the Doctor, calling him a traitor. The Doctor ignores this and asks the Old Silurian why they stayed in hibernation. The Old Silurians tells him the hibernation mechanisms was faulty but when the research center was activated, the energy surge woke them up. They now want to wake everyone and retake the Earth. The Doctor thinks this would provoke a war.

He now says that he could persuade the humans to make peace. The two species could live together, with the Silurians moving to the hot regions of the planet that man can't use. The Old Silurian seems moved by this idea. The Doctor presses the advantage, asking him to release the soldiers as a sign of good will.

Among the trapped soldiers, Roberts is carving on the wall. Hawkins tries to talk to him but Roberts assaults him, then returns to carving. In the Silurian base, the old Silurian uses his third eye on their computer. Suddenly, the cave wall opens in front of the Brigadier. They hurry to the surface.

The Young Silurian arrives, furious that the apes have been released. He insists the planet belong to them but the Old Silurian thinks the humans should be treated as equals. The Young Silurian stalks out, repeating his belief that they should wipe the humans out.

The Silurian Scientist is comparing images of apes to humans when the Young Silurian arrives, still furious that their leader wants to share the planet with apes. He wants to wipe them out and thinks the Old Silurian is no longer fit to lead. He threatens to kill the Scientist if he doesn't help him. The Scientist tells him they once used a plague on the apes when they would raid crops. They let Baker out of his cage. He tries to escape but they knock him out.

At the center, everyone is quiet, waiting for news. Lawrence is getting angrier by the minute. He sarcastically remarks that maybe they should send a search party after the search party. Dawson still thinks they should attack. The Brigadier suddenly returns. He tells his story, which Lawrence doesn't believe. Masters asks where the Doctor is.

The Old Silurians wants assurance that the humans will make peace. The Doctor can only agree to try. The Old Silurian tells him to wait and leaves, putting a forcefield over the door.

One of the Silurians leaves Baker lying in the cave and then wakes him from a distance. Baker sits up, looks around and then hurries away, clearly unaware of anything amiss.

The Old Silurian is angry that the Young Silurian has released the disease. He says the apes are civilized now but the Young Silurian insists this only makes them more dangerous. The Young Silurian says the Old Silurian is no longer fit to lead. The Old Silurian threatens to kill him if he defies him again.

The Doctor is looking at images of the Silurian civilization on their computer. The Old Silurian returns and tells him that Baker has been released, but infected with a disease. He insists this was not of his doing. He gives the Doctor a sample of the bacteria so that he can find a cure and then turns the Doctor loose.

The Brigadier is preparing another expedition when Baker returns. Baker tells them the Doctor has betrayed them. Masters asks Baker how he escaped, but Baker is evasive. He tells them they need to blow up the caves. But the Doctor arrives at this moment and tells everyone to move away from Baker. He tells Baker that he is very very ill. Baker tries to arrest him and the Brigadier moves to intervene, but the Doctor warns him off again. He tells Baker to look at his wrist. Baker reluctantly does and find a horrible rash on his arm. He collapses to the floor.

The Young Silurian is angry that the Old Silurian gave the humans the bacteria. The Old Silurian insists that two species can live together. The Young Silurian waits until his back is turned and then uses his third eye to kill him.

The Doctor wants to quarantine the research center, but this is only making Lawrence angrier. Masters is still skeptical, wondering why they should negotiate with the Silurians now that they've released the plague. But before the Doctor can respond, Liz arrives with dreadful news. Dr. Meredith has sent Baker to the local hospital over her objections. The Doctor is horrified and he and the Brigadier rush out to intercept the ambulance.

Masters gets ready to leave for London. He tells Lawrence the center will be closed and Lawrence is devastated at the news. As Masters gets up, he feels ill. But he thinks he's just tired and heads out to catch his car.

The Doctor and the Brigadier race to the hospital, desperate to contain the Silurian plague. But they are too late. As they roll up, Sergeant Baker collapses to the ground, the first victim of the plague.

A physician and nurse hurry out of the hospital to tend to the dead man, but the Brigadier forces them away at gunpoint. He agrees to stay and quarantine the hospital while the Doctor goes back to the lab to work on a cure.

The Doctor is looking at slide, trying to puzzle out what they are dealing with. Liz arrives and confirms they have pumped antibiotics into everyone at the research center as a precaution. She wants to inject the Doctor as well. He protests that he is not human, but relents eventually. The Brig arrives and assures him that he has cordoned off the hospital and ordered a sophisticated microscope to be sent it. Liz tells him that Dawson, the doctor and nurse at the hospital and the ambulance driver have all caught the disease. She also mentions that Lawrence and Masters can not be found, then leaves to locate and inject them.

The Brigadier worries that the disease can't be contained. The Doctor is no mood to chat though and wants to be left alone to work. But before he can get started, Liz returns and tells him that Masters left for London. The Brigadier rushes out to stop him.

Masters arrives at a London train station, feeling progressively worse. He leaves and walks down the street, getting more and more disoriented. He gets into a taxi just as the police arrive to apprehend him.

A huge microscope is delivered to the Doctor as he and Lethbridge-Stewart worry about Masters. The Brigadier wonders what the Silurians will do. The Doctor speculates that they will just wait for the plague to do its work. The Brigadier again wonders if they can make peace with the Silurians. But the Doctor shoos him away again, eager to continue his work.

The Young Silurian stands over the body of the old one and announces that he is now the leader. After asking them to remove the body, he wonders if the disease has wiped the humans out yet. The Scientist reminds him that the apes are more advanced now. They may resist the disease or even cure it. The Young Silurian scoffs but the Scientist shows him an image of the Doctor working in the lab. He reminds the Young Silurian that this ape is far more intelligent than the others.

Lawrence finally turns up, strolling into the office where Liz is working. He's frustrated and wants to talk to the Brigadier. He's angry the center is at a standstill because of the injections. He refuses to get injected, thinking the epidemic is imaginary. Liz points out that Baker is dead, but Lawrence will admit nothing and leaves. The Brigadier arrives just as the phone rings. He finds out that the police missed Masters at the station.

Masters stumbles out of his taxi, getting ever more disoriented. The ticket collector at the train station begins to feel ill.

In the caves, a UNIT soldier hears a noise and looks into the darkness. He tries to call for help but a Silurian steps out and kills him. Hawkins rushes to the scene. The Silurian kills another man and Hawkins opens fire on it, chasing it away. Back in the Silurian base, the Young Silurian notes the soldiers are guarding the cave and is frustrated the disease hasn't killed them yet. He decides to capture the Doctor to prevent him from finding a cure.

A huge sample of drugs is brought to the Doctor. He is hopeful that one or some combination will cure the Silurian plague and asks Liz to get a blood sample for him to work on.

The tension continues to grow. While the Doctor works and the Brigadier organizes the quarantine, people are dropping all over London from the disease. Masters is on his last legs as the police close in on him. Liz brings the Doctor blood samples and he begins to try out the drugs on them. The Brigadier assures the Ministry that the Doctor and Liz are working as hard as they can.

The Doctor is slowly losing confidence, but Liz is optimistic that they've narrowed down the solution. Liz swoons a bit, feeling the effects of the drugs. The Doctor tells her to inject herself again. She wants to inject the Doctor as well but he insists he's fine.

Liz meets with the Brigadier again and he tells her that Masters is dead and the plague is breaking out all over London. Lawrence stagger into the office, his face covered in the disease rash. They ask him to go the infirmary and he loses it. He shouts that he was an important man before UNIT came and now his career is in ruins. He thinks everything that happened was a plot to get rid of him. He yells at them to leave and then attacks the Brigadier before collapsing to the ground, dead.

With a cry of "Eureka!", the Doctor realizes he has found it -- the cure to the Silurian plague!

The Brigadier wearily reports that the first case abroad has appeared in Paris. Liz worries that if they can't contain it to Britain, the world is doomed. But the Doctor bursts in and tells them he's found the cure. He and Liz hurry to the infirmary to try it out on a patient.

The Scientist leads the Young Silurian to the point in the cave closest to the human base. They use their third eyes to melt a passage through the rock.

Liz tells the Doctor that the ambulance driver is responding to the drugs. He rushes back to the lab to write down the formula.

Hawkins reports to the Brigadier that the Silurians are advancing out of the caves, attacking the soldiers. The Brigadier hurries back to his office where the phone is ringing. But it's just a tabloid reporter trying to find out what's going on.

The Doctor is working on a cure to the deadly Silurian plague when the wall in back of him begins to glow. Suddenly, the Silurians swarm in and knock him to the ground with their third eyes.

The Brigadier is trying desperately to stay on top of the situation with both the plague and the Silurians. The Silurians are advancing against his troops but at least the plague patient has been cured.

The Silurians drag the Doctor into the passageway they've burned into the research center. A soldier come down the corridor and see them but the Young Silurian kills him before he an raise the alarm. The Silurians hustle the Doctor away, the Young Silurian closing the passageway with his third eye.

The Brigadier and Liz are handling frantic phone calls. Dr. Meredith is getting impatient in the infirmary, wanting to give the plague cure to the rest of the patients.

Captain Hawkins is on his way to report to the Brigadier when he finds the dead soldier and a bizarre scorched pattern on the wall.

The Ministry calls again and Liz decides to see if the Doctor has the formula for the cure written down yet. And the Defense Ministry can't make up its mind about whether to send the Brig more soldiers.

Liz finds the Doctor gone. She start frantically looking through his notes, trying to find the cure for the disease. The Brigadier arrives, demanding to know where either the formula or the Doctor are. Liz tells him she has no idea where he's gone and she has no idea which is the correct formula. Hawkins then arrives and informs them that there has been some kind of raid. They leave as Liz continues to look through the Doctor's notes. With a sudden smile, she finds it and picks up the phone to call the hospital.

The Young Silurian is interrogating the Doctor and asks how soon the apes will die. The Doctor tells them that the disease has been cured as their leader wanted. The Young Silurian smugly informs the Doctor that he is now the leader.

The Brigadier tells LIz that the Silurians have broken in and abducted the Doctor. He orders Hawkins to evacuate the research center and send every soldier that can be spared to the caves. Liz tells the Brig she's given the formula to the hospital. She wants them to raid the Silurian base to rescue the Doctor but the Brigadier reminds her that they are barely holding off the Silurians as it is.

The Young Silurian now decides that they will make the entire planet uninhabitable for humans by connecting the "disperser" to the human generator. The Doctor informs him that the reactor has been shut down, but they are undeterred. They decide to storm the center and and force the Doctor to turn the reactor back on.

Liz and the Brigadier are still in the research center when the lights turn off and the phones go dead. The Brigadier suddenly realizes that by sending all his men to the caves he has left the research center defenseless.

The Silurian Scientist tries to wake more Silurians but there is not enough power.

The Brigadier and Liz try to use the elevator but it is jammed. Suddenly, the walls glows and the Silurians walks in with the Doctor. They force the Brigadier to put down his weapon. Hawkins is in the lab and overhears and begins to sneak up on the Silurians. They have decided to go ahead and kill the Brigadier when Hawkins attacks. They kill him easily but the Doctor threatens not to help them if they kill anyone else. The Young Silurian tells him to activate the generator or they will kill everyone.

The Silurians break into the reactor room and immediately start killing the scientists. But the Doctor yells at them to stop. He then calmly tells the scientists to re-activate the reactor. The Brigadier asks him what he's doing and the Doctor replies that he is saving their lives. The Silurians wheel in a large cumbersome device and roll it up to the generator. The Doctor asks what it does. At first they won't tell him, but then they identify it as a disperser that will use microwaves to disperse the van Allen belts. Liz tells the Brigadier this will make the Earth uninhabitable to humans, but fine for reptiles.

In the cave, the Silurians try reviving more of their people, but the power is still too low.

The Silurian Scientist tells the Doctor to turn the power up further. The Brigadier wants him to stop but Liz tells him to trust the Doctor. The Doctor tells Liz to lower all the power rods at once when he tells her. They start the reactor and the power slowly builds. The Silurians turn on the disperser. The Young Silurians wants more power and the Doctor tells Liz to drop all the rods. She does and the power spikes, blowing up the disperser. The Young Silurian is enraged. He tells them to turn off the reactor but the controls blow up before they can.

The Doctor tells the Young Silurian that the reactor is overloaded and will soon explode with a massive amount of radiation, making the area uninhabitable for decades. The Young Silurian decides to send his people back into hibernation. He will leave the humans to die. The Silurians withdraw, leaving their ruined disperser behind.

The Brigadier starts to congratulate the Doctor, but the Time Lord replies that he wasn't really bluffing. They need to get out before the reactor explodes. But the Brigadier informs him that the lift is jammed and they can't escape.

In the caves, the Silurians are hurriedly putting their race back into hibernation. The Silurian Scientist realizes, however, that because of the fault in the hibernation mechanism, one of them must stay awake. The Young Silurian volunteers to do it. He will set the controls to wake everyone in 50 years. He tells the Silurian Scientist to lead when they awake and wipe out all the humans.

The Doctor is working frantically to shut down the overloaded reactor. Nothing is helping until he fuses the controls of the neutron flow which blows the console and shuts down the reactor.

Some time later, the Doctor returns to the Silurian lair. The Young Silurian appears and realizes he has been tricked. He will now force the humans to revive them -- after he kills the Doctor. His eyes glows with menace but the Brigadier appears and guns down the Young Silurian. He asks the Doctor what he thinks he's doing. The Doctor wants to examine the hibernation machinery. He thinks that if he can wake them up one at a time and reason with them, there can still be a peaceful solution He marvels at the wealth of knowledge that is in the Silurian base. The Brigadier looks strangely guilty and reticent.

Later that day, the Doctor returns to the research center. He reminds the Brigadier that no one is to go into the base. He and Liz will return soon to begin a full exploration. The Brigadier is quiet and strangely eager to see them off. After they leave, he asks a corporal if everything is set up. When told it is, he instruct the young soldier to wait until the Doctor and Liz are away and then set off the explosives and seal the Silurian base for good.

The Young Silurian has stubbornly clung to life and has regained consciousness. He staggers to his feet and tries desperately to wake one of his fellow Silurians using his third eye for power. It glows weakly as he gasps for breath.

On the Moors, Bessie has broken down. While Liz looks on, the Doctor uncorks a phial of vile-looking liquid. He pours it into Bessie's radiator. Gurgles move through the car, a few drops fall out of the tailpipe and she starts up. But before the delighted Doctor can get going, an explosion roars out.

In the Silurian base, the mortally wounded Young Silurian is crushed as the cave collapses around him.