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...