Exiled to Earth and now working for UNIT as their scientific advisor, the Doctor is summoned to an underground research centre at Wenley Moor to investigate a series of inexplicable power losses. Initially suspecting sabotage, he soon discovers that the nearby cave system conceals a colony of an ancient reptilian race called the Silurians. Awaking from a hibernation which has lasted millions of years, they are now intent on reclaiming the Earth from Mankind...