The Silurians (Homo reptilia) were Earth's first intelligent inhabitants, cold-blooded reptilians with a lifespan of over three centuries, who developed a civilisation at least 300 million years before human beings evolved. Silurian astronomers predicated the arrival of a minor planet would devastate Earth, and so the species went into hibernation, with one genetic line being left as caretakers while the others slept. Now they are awake and ready for war. And only the Doctor can stop them.