The Doctor plans to capture a dinosaur and discover who is bringing them through time.
While the soldiers fight the tyrannosaurus, the Doctor and Sarah escape and hide in a garage. A medieval peasant from the days of King John appears, then disappears in a time eddy. Soldiers enter the garage, accompanied by the Brigadier.
Back at UNIT HQ the Brigadier explains the situation to the Doctor. Dinosaurs appear and disappear, the city has been evacuated, and looters run rampant. The Doctor is introduced to Finch, who is in charge of the operation. Finch doesn’t think the Doctor will be much help. Sarah talks with Mike Yates, who needed some time off after the experience with the giant maggots.
A stegosaurus has been trapped, and the Doctor wants it taken alive. However, it disappears in a time eddy that makes time run backwards. The people affected would not remember it happening. The Doctor suspects someone is deliberately bringing the dinosaurs to London — and in a hidden laboratory a pair of scientists, Butler and Professor Whitaker, are operating the time technology causing the situation.
They are aided by Mike Yates, who feels the Doctor could help them achieve Operation Golden Age, but Whitaker is unconvinced. He tells Mike to sabotage the stun gun the Doctor is building to use on the dinosaurs.
The Doctor believes the dinosaurs are a distraction by someone who needs London evacuated. When a brontosaurus appears, he heads out to capture it. Mike sabotages his stun gun, and it doesn’t work on the brontosaurus. The Time eddy takes away the brontosaurus, and a tyrannosaurus appears behind the Doctor.