The TARDIS materialises in a deserted London. The Doctor and Sarah see a car but it won't stop so they follow it and find its driver, Phillips (Gordon Reid), robbing a jeweller's. The looter runs off but a screech of tyres and a bloodcurdling roar brings them outside to find the robber dead and the car's bonnet smashed in. The Doctor and Sarah follow a land rover to a garage and disturb two more looters (Terry Walsh, Alan Bull) and are attacked by a pterodactyl. They escape by taking the land rover. They are stopped by UNIT soldiers who arrest them and take their photographs in preparation for a military trial. 

UNIT are trying to find out why dinosaurs have been appearing all over Central London but the Brigadier (Nicholas Courtney) is no nearer a solution. In overall charge is General Finch (John Bennett). Benton (John Levene) recognises the photographs of the Doctor and Sarah and the Brigadier sends a despatch rider to fetch them. 

Lieutenant Shears (Ben Aris) sentences the Doctor, Sarah and another looter, Lodge (Trevor Lawrence), to be held in a military detention centre. The Doctor escapes and he and Sarah hide in a warehouse where they meet a peasant (James Marcus) from the time of King John who fades away in a time eddy. The Brigadier arrives and they all return to UNIT HQ. 

Finch won't listen to the Doctor's theories. The Doctor believes that someone in Central London is behind the dinosaurs and builds a stun-gun to knock out a dinosaur so that he can monitor it when it vanishes and trace the source of the power used to transport it to the present day. 

A scientist called Whitaker (Peter Miles) is engineering the appearances and working with a man called Butler (Martin Jarvis) to ensure that Central London remains empty of people. Captain Mike Yates (Richard Franklin) of UNIT is in league with them and he warns Whitaker of the Doctor's plans. Whitaker gives him a small device which will render the Doctor's stun-gun ineffective. 

A brontosaurus appears and the Doctor goes to stun it. Yates sabotages the gun but the brontosaurus vanishes and a tyrannosaurus appears instead. Yates removes the device from the gun and stuns the creature before it can harm the Doctor. The beast is moved to a hangar and the Doctor places his detection devices around it. Whitaker tells Yates that he must sabotage the Doctor's equipment. 

Sarah remembers hearing about Professor Whitaker from a story she worked on but no-one will follow up her suspicions. Sir Charles Grover MP (Noel Johnson), who is still in London, claims Whitaker was a crank but Finch seems interested and also arranges for Sarah to obtain a special pass so that she might take photographs of the captured tyrannosaurus. While she is doing so, it revives and she finds herself trapped in the hanger. She is rescued by the Doctor. It transpires that the chains that held the creature had been cut and that all the Doctor's equipment had been sabotaged. The Doctor intends to build a portable device to try and locate the power source, probably a small nuclear reactor. Sarah remembers about nuclear reactors being built into government bunkers and goes to see Grover who helps her to locate the correct paperwork. The reactor is actually under the building in which Grover is based - he has tricked Sarah and takes her in a concealed lift to the underground base. There she is hypnotised with flashing lights. 

When she wakes, she appears to be in a space craft some three months out from Earth, en route for a planet dubbed New Earth. She meets 'Mark' - in reality athlete John Crichton (Terence Wilton) - 'Ruth' - Lady Collingford (Carmen Silvera) - and 'Adam' - novelist Nigel Castle (Brian Badcoe). There are 200 people on seven ships heading for the planet. Sarah feels the bump sustained in the hangar on her head, and realises that all this is untrue. Ruth determines that Sarah must be re-educated and she is locked in a room which plays a continuous film of the pollution on Earth. 

The Doctor travels in his new car - a futuristic hovercraft vehicle - with his tracking device until he locates the power source in a London Underground station. He sees Butler enter and use a lift disguised as a janitor's cupboard to descend into the base. The Doctor follows him but is detected by Butler and Whitaker and routed back to the lift. When he returns to the Underground station, the Doctor is attacked by another pterodactyl before he manages to escape. 

Grover plans to discredit the Doctor and so Whitaker places a call to UNIT, telling the Doctor that he will meet him in the hangar. When he arrives, Whitaker summons a stegosaurus and then Finch bursts in, accusing the Doctor of being behind the appearances. 

On the spacecraft, Ruth tells the others that Sarah will have to be eliminated if she cannot be re-educated. Mark goes to warn Sarah. Sarah escapes and after discovering that the flight equipment on the spacecraft does nothing, she proves to Mark that it is all a fake by leaving through the airlock. She returns to UNIT HQ. There is no-one there but Private Bryson (Colin Bell) as Finch has given orders that the Doctor, whom Benton allowed to escape, be shot on sight, and the Brigadier and Benton have gone to try and find him first. Finch arrives and Sarah confides in him. She takes him back to Grover's office where he pulls a gun on her and they descend to the base once more. 

Grover orders a final wave of dinosaurs to ensure that no-one remains in London. He also gets Finch to order all the UNIT and government troops to pull out. His plan is to roll back time on Earth so that those within a protective field of influence will arrive at an earlier 'golden age'. 

The Doctor meets up with the Brigadier and they return to UNIT HQ. There, Yates threatens them with a gun, but Bryson inadvertently distracts him and the others overpower him. The Doctor and the Brigadier head for the Underground station with explosives and rope. 

Sarah is locked in a store room but escapes through a ventilation shaft. She returns to the fake spaceship but Ruth has her and Mark locked up. Adam is suspicious and sends a message to Grover who is allegedly on one of the other ships. He arrives in a space suit and admits his plan to Sarah and Mark, unaware that Adam is listening at the door. When he has gone, Adam releases Sarah and Mark and they head for the airlock. Sarah opens it and the rest of the passengers follow her out, determined to confront Grover. 

The Doctor and the Brigadier blow up the lift entrance. While the Doctor descends using the rope, the Brigadier gets Benton to send reinforcements. Benton overpowers Finch, who was holding him at gunpoint, and complies with the Brigadier's request. In the base, the Doctor knocks Butler out and then confronts Grover and Whitaker as do Ruth and the others. The Brigadier and Benton arrive and Whitaker takes the opportunity to start his machine. Time stands still for everyone except the Doctor who manages to switch it off. He reverses the polarity but then Grover lunges at the controls and, as he and Whitaker struggle, they and the machine vanish into the past. 

The Brigadier grants Yates extended sick leave and the opportunity to retire quietly and the Doctor tries to persuade an initially reluctant Sarah to go with him to the planet Florana.

_{Synopsis from Doctor Who: The Third Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}