The Doctor and Sarah arrive in 1970s London to find that the city has been evacuated, due to the mysterious appearance of dinosaurs. The monsters are being brought to the present by a disaffected scientist, using a time machine nicknamed the Timescoop, as part of a plan to revert London to a utopian pre-technological age. The plan calls for Central London to be depopulated prior to being moved back through time, so that only an elite group (who will re-found the human race in the remote past, along more ethical lines) will make the trip.
This plan, known as Operation Golden Age, is secretly being supported by key personnel within the British Government - and even within the United Nations Intelligence Taskforce, which is ostensibly combatting the menace.