The Doctor and Sarah return to 20th Century London to find it deserted. Initially arrested as looters, they are soon back with UNIT. The Brigadier explains that central London has been evacuated due to the unexplained and random appearance of prehistoric monsters. The Doctor and Sarah discover that the monsters are being brought through time by two scientists, Whitaker and his henchman Butler, as part of a conspiracy to clear London of people.
The Doctor's investigations are deliberately hindered first by Captain Yates and then by the Army's General Finch, both of whom are involved in the conspiracy. Sarah visits Sir Charles Grover, the Government Minister in charge during the emergency, but discovers that he too is involved.
Captured and hypnotised, she revives to find herself apparently on board a spaceship en route to colonise another world. She soon realises that the ship is just a mock-up and those on board, led by a number of elders, have been duped.
The conspirators in fact intend to use a time machine to return London to a 'Golden Age' before the Earth became polluted, so that they and the would-be colonists can start civilisation afresh.
The Doctor and the Brigadier mount a raid on their underground headquarters. Grover makes a last-ditch attempt to operate the time machine, but the Doctor has changed the settings and he succeeds only in sending himself and Whitaker back to the era of the dinosaurs. The Brigadier arranges for the misguided Yates to be given a chance to resign quietly from UNIT.