The Doctor and Sarah returns to present-day Earth, where they find London has been evacuated, and Dinosaurs are on the loose and they are mistaken for looters. Assisting U.N.I.T in the emergency, The Doctor and Sarah discovers corrupt politician Sir Charles Grover MP, renegade scientists Butler and Professor Whitaker and military officer General Finch are using a time machine to bring the dinosaurs from the past to the present as part of they scheme to bring the human race back to a "Golden Age" before the world was polluted.

Sarah attempts to find a way off the spaceship while the Doctor tries to track down the source of the time displacements.