An experimental nucleic power research centre built into a network of caves in Wenley Moor is experiencing mysterious power drains and a high incidence of mental breakdowns. UNIT are called in to investigate, and the Third Doctor and Liz meet Brigadier Lethbridge-Stewart at the plant. While exploring the caves, one of the workers at the centre was killed, with wounds that look like giant claw marks, and his companion's mind has been traumatised to the extent that he can only scrawl primitive cave drawings of reptilian creatures on the walls of the ward. Lawrence, the Director, resents UNIT's presence and feels that it will interfere with the working of the plant, which is trying a new process to convert nuclear energy directly into electric power. Off in a corner, Dr Quinn, the Deputy Director, argues with Miss Dawson, his assistant, when she protests that they should stop "them" from taking the power. Major Baker, the security chief, believes there is a saboteur in the centre, and the Doctor discovers that the logs of the nuclear reactor's operation have been tampered with.
When the Doctor makes his way into the caves, he is attacked by a dinosaur-like creature, but it is called off by a strange whistling sound. The Brigadier decides to explore the caves with armed men. Baker fires at a humanoid figure he spots in the caves, and is attacked by the dinosaur-like creature, but it is again called off by the same sound. The Doctor returns to the centre with Baker, taking samples of the blood that was drawn when Baker fired at the humanoid. Examining the blood, the Doctor finds similarities to the "higher reptiles". In the meantime, the humanoid has reached the surface and stumbles into a barn to hide. Quinn goes into the caves to a hidden base, where he demands the knowledge he was promised. He is told that he must first see to the wounded humanoid and is given a signal device which emits the sound heard earlier.
The humanoid is discovered by a farmer and his wife, and attacks them. The farmer dies of a heart attack, but the wife survives and identifies her attacker. Liz, investigating the barn, turns to see a reptilian biped, who knocks her unconscious and flees. When Liz awakes, she tells the Doctor and the Brigadier, and the latter orders a manhunt across the moor, just as Quinn arrives at the barn. Making an excuse, he leaves and summons the humanoid with his device. When the Brigadier and the Doctor discover the humanoid's tracks, they end in tire marks.
The Doctor goes to Quinn's cottage, and notes that it is remarkably hot, like a reptile house. Quinn replies that the thermostat is broken. Quinn does not react well to the hints the Doctor is dropping or to his veiled attempts to get him to co-operate, and asks the Doctor to leave. Breaking into Quinn's office, the Doctor finds a globe that depicts the Earth's continents as they were 200 millions of years ago, which the Doctor identifies as during the Silurian epoch. Back at the cottage, Miss Dawson tries to persuade Quinn to tell the Doctor everything, but Quinn is adamant that he will keep the wounded Silurian captive until he is given the advanced scientific knowledge he wants. Later, when the Doctor goes to the cottage to once again try to reason with Quinn, he finds the scientist dead in his chair. The Doctor retrieves the signal device from Quinn's body and is surprised by the Silurian, who is still there. The Doctor attempts communication, only for the Silurian to run away.
Baker, still convinced that the answer lies in the caves, overpowers the UNIT man guarding the sickbay and enters them by himself. He is soon captured by the Silurians and interrogated as to the capabilities of the humans. The Doctor and Liz explore the caves following Baker's route and open the entrance to the Silurian base with Quinn's signal device, where they find Baker in a locked cage. He tells them that they must warn the surface. The Doctor and Liz leave undiscovered, but not before they see a Silurian being revived from hibernation by a machine, explaining the energy drains that the reactor has been experiencing. Meanwhile, Masters, the Permanent Under-Secretary in charge of the centre, arrives for a personal inspection, and Lawrence complains to him about UNIT's interference. The Doctor decides to tell them all about the Silurians in the caves, urging a peaceful contact instead of the Brigadier's proposed armed expedition. However, this falls on deaf ears when Miss Dawson reveals that Quinn was killed by the Silurian he held captive.
The Doctor goes to warn and reason with the Silurians, but they put him in a cage as well. An older Silurian seems to be willing to listen to the Doctor, but a younger one wants to destroy the humans and retake the planet. The UNIT troops soon find themselves trapped, with stone walls inexplicably appearing to block off their passage. The younger Silurian attacks the Doctor psychically with a glowing third eye, but the older one puts a stop to it. The older Silurian tells the Doctor how their race retreated underground when they saw the Moon approaching Earth millions of years before. The hibernation mechanism malfunctioned, and they did not revive until a new power source — the nuclear reactor — was discovered. The Doctor manages to persuade the older Silurian to release the Brigadier and his men, but the younger Silurian orders Baker infected with a virus before he is released. When the older Silurian discovers this, he releases the Doctor, giving him a canister of the virus so he can discover a cure. The younger Silurian kills the older one for this act.
At the centre, the Doctor warns everyone to stay away from Baker, who collapses with the virus. Masters, however, decides to return to London, unaware that he has also been infected. Baker is taken to a local hospital without the Doctor's knowledge and dies there. The Brigadier holds Baker's doctor and nurse at gunpoint to prevent them from leaving and spreading the virus while the Doctor returns to the centre to work on the cure. The Brigadier and Liz try to warn London, while all of the centre's staff are inoculated with a stopgap vaccine. All, that is, except for Lawrence, who refuses. Masters, in the meantime, has reached London and eludes the search parties desperately looking for him. The virus begins to spread and the deaths begin, the infection even reaching France. Lawrence eventually dies from the virus, complaining in his last moments that the affair has ruined his career.
The young Silurian decides the Doctor must be killed before he finds a cure. The Doctor has indeed found one, and as he writes down the formula, the Silurians attack the centre and stun him with their third eyes, taking him away. Liz, however, discovers the formula and it is soon being mass produced and distributed. The Silurians have a back-up plan, however. They intend to use a weapon to destroy the Van Allen Belt and make the Earth's environment hostile to humankind, and will force the Doctor to connect the reactor to provide power to the weapon. UNIT troops are lured into the caves and commence a battle with the Silurians while the younger Silurian takes the Doctor to the reactor control room along with Liz and the Brigadier. The Doctor, however, overloads the reactor and tells the younger Silurian that the area will be irradiated for at least 25 years. The Silurians disengage from the battle with UNIT and reenter the caves to hibernate until the danger has passed. Since the mechanism is faulty, the younger Silurian will stay awake to operate it and sacrifice his life. The Doctor and Liz, meanwhile, repair the reactor, and go to the underground base, where the younger Silurian realises he has been duped into sending his race back to sleep. He attacks the Doctor in a rage, but is killed by the Brigadier.
Later, on Wenley Moor, the Doctor tells Liz that he proposes to revive the Silurians one by one and try to reach a peaceful compromise between them and humanity. However, the Brigadier has other orders, and the Silurian base is blown up. The Doctor is horrified at this act of genocide, but Liz tells him that the Silurians would never have agreed to come to terms. The Doctor, still disgusted, drives off.
Doctor Jon Pertwee (Third Doctor)
Companion Caroline John (Liz Shaw)
Nicholas Courtney —Brigadier Lethbridge-Stewart
Fulton Mackay — Dr Quinn
Peter Miles — Dr Lawrence
Ian Cunningham — Dr Meredith
Norman Jones — Major Baker
Thomasine Heiner — Miss Dawson
Ian Talbot — Travis
Geoffrey Palmer — Masters
John Newman — Spencer
Bill Mathews — Davis
Roy Branigan — Roberts
Gordon Richardson — Squire
Nancie Jackson — Doris Squire
Brendan Barry — Hospital Doctor
Paul Darrow — Captain Hawkins
Richard Steele — Sergeant Hart
Alan Mason — Corporal Nutting
Harry Swift — Private Robins
Derek Politt — Private Wright
Dave Carter — Old Silurian
Nigel Johns — Young Silurian
Pat Gorman — Silurian Scientist
Dave Carter, Pat Gorman, Paul Barton, Simon Cain, John Churchill — Silurians
Peter Halliday — Silurian Voices
Writer Malcolm Hulke
Director Timothy Combe
Script editor Terrance Dicks
Producer Barry Letts
Executive producer(s) None
Production code BBB
Series Season 7
Length 7 episodes, 25 minutes each
Originally broadcast January 31–March 14, 1970