The Silurians (actually a misnomer - they are more properly called "Eocenes") are tall, bipedal reptilian beings, with clawed hands, webbed feet and green scaly skin. A neck ruff drapes across their shoulders, and down their spine is a row of small, pointed dinosaur-like plates. They have yellow eyes and oval-shaped mouths. One good look at a Silurian causes intense reactions of fear in most human beings, paralyzing them with terror or even killing them from heart failure. Atop the Silurians' heads is a red third eye, which is very functional. They can see through it (at times a triple image, other times a fused view) and use it as a power source, in which case it pulses with light. The third eye can kill both humans and other Silurians, revive people from unconscious (or drugged?) states, create forcefields, burn through rock and walls and reform them (although it leaves a scorch mark) and power devices like the doors of the Silurian city, the doors to their cells, a viewing device that seems to be able to see anywhere, and a machine that can rearrange the structure of the cave system. Being reptiles, they are naturally cold-blooded, meaning that they like the heat. They are also susceptible to bullets.
The Silurians later undergo a change (or, perhaps, there is a different group of Silurians with different physical features). Future Silurians have a bony chest plate instead of the neck ruff, and their eyes are golden and catlike. The third eye of these Silurians still glows red, but only in time with their mechanized-sounding voices; they never use the third eye for another purpose.
Millions of years ago, the Silurians ruled the planet Earth. Their advanced technology detected a small planet approaching Earth, and they calculated that it would draw off the atmosphere, destroying all life. In the time they had left, the Silurians built hibernation chambers and suspended their lives until the atmosphere would return.
Their predicted catastrophe never occurred. Instead of drawing off the Earth's atmosphere, the planet settled into Earth's orbit, becoming its new moon. Worse, the Silurians' hibernation mechanism was faulty, and failed to function until a new energy source appeared: an atomic research centre built deep beneath the Earth's surface. The centre's attempts to produce safe, inexpensive electrical power from nuclear energy, using a device called the Cyclotron, revive some of the Silurians from their deep hibernation. Unable to make their own power, the Silurians then proceed to drain power from the Cyclotron to revive more of their comrades. They are aided by one of the centre staff, who wishes to help them in exchange for their knowledge.
The Silurians are very technologically advanced. They constructed a city deep in the Wenley Moor cave system and protected it with traps, such as a pool of liquid that appears to be acid. They created a summoning device that can be used to control their dinosaurs, open doors, and even summon other Silurians. The device emits a signal of three cascading notes, which speed up as the Silurian approaches. They can control the movements of other beings with some kind of red beam. They are also able to speak and understand English. Future Silurians possess a device called a manipulator, with which they can bypass the need for a sync operator and interface directly with a computer. They move about in battle cruisers, which can travel through the sea and use a particle suppressor to deflect weapons systems back on the aggressor.
The Silurians' mere presence begins to cause problems. Over the course of three months, the staff working in the Cyclotron suffer an unprecedented number of nervous breakdowns, and the Silurians' power-siphoning causes frequent power outages that threaten the safety of the centre. When two men go spelunking, one is killed by a dinosaur controlled by the Silurians; the other is so scarred by the fear that his mind is thrown back millions of years, leaving him terrified and able only to draw cave-pictures on the walls of the hospital unit.
The Silurians decide that it is time to reclaim the Earth for themselves. After all, the humans are nothing more than crude apes that have taken over their planet. With this in mind, they set out to collect as much information as possible about the current state of the Earth from the captives they manage to collect. The Doctor convinces an Old Silurian that his race doesn't have to take over the Earth; they can co-habitate the planet with the humans, living in the hot areas in which the humans have no interest. Although the Old Silurian is prepared to accept the Doctor's proposal, a Young Silurian is not. He kills the Old Silurian and infects a captive with a deadly bacterial plague that they had once used on the apes that ruined their crops. The Young Silurian hopes that this disease, highly contagious and quick to kill, will soon wipe out the humans. The bacteria produces a reddening of the skin, boils, scabs and peeling flesh; unable to contain it to the research centre, the Doctor and company watch in horror as it spreads through London and into Paris. But the Doctor is able to concoct an antidote before things have gone too far.
The Silurians' only remaining recourse is to launch an assault on the Cyclotron itself. They bring with them a molecular disperser, which will convert the energy of the nuclear reactor into microwaves which will disperse part of the Van Allen belt. The Silurians claim that they can keep the reaction from destroying all life on the planet, but Liz realizes that they intend to allow more solar radiation into the atmosphere to heat the surface of the planet, making it intolerable for humans but a place where the Silurians can thrive. With her help, the Doctor overloads first the disperser, then the reactor. Faced with a nuclear explosion, the Silurians retreat to hibernate until the radiation has dispersed, and the Doctor is able to power the reactor down.
The Doctor wants to revive the Silurians again, to discuss the situation with them reasonably and win them over to his way of thinking, but the Brigadier intends to take no chances. He has the UNIT troops lay charges, and then he blows up the Silurians' base, killing them all.
There is an indication that the third or fourth Doctor encounters the Silurians again. During this adventure, the Doctor meets a Silurian Triad, which wish to establish peaceful relations with Earth. But again, something goes wrong, and the Doctor assumes that the Triad are all killed. But in fact, one - Icthar - survives, and goes on to revive a group of Sea Devil warriors in order to take over a Sea Base and its proton missiles. Icthar wants to start a war among the humans, causing them to annihilate themselves and leaving the Earth free for Eocene reinhabitation. When the Doctor finds that he cannot persuade them against their chosen course of action, he has no choice but to poison them all with hexachromite gas, a substance lethal to marine and reptile life.
