Jamie manages to shut the TARDIS doors and the ship materialises in space where a strange web-like substance begins to cover the exterior. The Doctor manages to free the ship and they arrive on Earth. 

Julius Silverstein (Frederick Shrecker) receives a visit at his museum from Professor Travers (Jack Watling) who wants back a Yeti robot that he brought from Tibet some thirty years earlier. Silverstein refuses to sell or to listen to Travers' warnings - Travers has managed to reactivate a control sphere which has gone missing - and Travers' daughter, Anne (Tina Packer), arrives to take the Professor home. After they have gone, the control sphere enters the museum and reactivates the Yeti which kills Silverstein. 

The TARDIS has arrived in Covent Garden Underground station in London. The Doctor, Jamie and Victoria find the station deserted and the entrance gates locked. They return to the tunnels where the Doctor discovers a group of soldiers laying cables. He follows the cables to Charing Cross where they are connected to crates of explosives. The Doctor watches from hiding as two Yeti (Gordon Stothard, John Levene) spray web over the crates. 

Jamie and Victoria are discovered by Staff Sergeant Arnold (Jack Woolgar) and taken to Captain Knight (Ralph Watson) at the Army's underground base at Goodge Street. There they meet Anne Travers and a television reporter, Harold Chorley (Jon Rollason). They discover that Captain Knight is about to blow up the tunnel and tell him about the Doctor. 

The explosion is neutralised by the Yeti's web. Knight sends Arnold out with a patrol to find out why the tunnel has not collapsed and also to locate the Doctor. They find no sign of him. An ammunitions supply truck is attacked by Yeti (Gordon Stothard, John Levene, John Lord, Jeremy King). Jamie and Victoria meet Professor Travers who is amazed that they have not aged. Jamie goes with Arnold to find the Doctor. 

Victoria stays with Travers and his daughter, but overhears Anne questioning her father as to the possibility that the Doctor is controlling the Yeti. Victoria decides to go in search of her friends. Corporal Blake (Richardson Morgan) and Craftsman Weams (Stephen Whittaker) report that the web is once more moving through the tunnels. 

Jamie and Arnold meet up with Knight and his men who have been attacked by more Yeti. While Knight and his men head back to base, Jamie and the ammunitions truck driver, Evans (Derek Pollitt), follow a Yeti which is carrying a transparent pyramid. They end up trapped between the web and the Yeti but escape when Evans shoots the pyramid. 

Victoria finds the Doctor and the new base commander Colonel Lethbridge-Stewart (Nicholas Courtney) who have survived the Yeti's attack on the ammunition truck. They return to Goodge Street. The Doctor sets to work with Travers as the base is quickly being cut-off. Lethbridge-Stewart and Knight plan to blow up the tunnel to halt the web but someone lets a Yeti into the explosives store and the charges are rendered useless. 

A patrol of soldiers is sent in an attempt to salvage explosives from the ammunition truck, but their way is blocked by web. Professor Travers is kidnapped by a Yeti while everyone else is occupied. Whilst the Doctor and Anne work on a way to control one of the Yeti spheres, Lethbridge-Stewart plans to get the TARDIS and bring it to the HQ by trolley. The plan fails and Sergeant Arnold disappears. 

The Colonel leads a squad of men above ground so that the Doctor can gather some essential electronic components from a nearby shop. The Yeti attack and kill Knight and leave a small model Yeti on his body. Only the Colonel and the Doctor survive to return to the HQ. As the group ponders what to do next the Yeti burst in with Travers, whose mind has been taken over by the Great Intelligence. 

Travers/the Intelligence orders the Doctor to surrender himself within twenty minutes to have his memories and knowledge drained, or his friends will be killed. To make even more sure of the Doctor's co-operation, Victoria is taken to the Intelligence's control centre at Piccadilly Circus. Once there the Intelligence releases Travers from its control. Travers and Victoria manage to make contact with Arnold who has been wandering in the tunnels. 

The Doctor has little choice but agree to the Intelligence's terms, but before he heads for Piccadilly Circus he and Anne finish work on their control sphere which will now make its Yeti follow their voice commands. They have also made a device that will block the control signals of the other Yeti. At Warren Street they insert the adapted sphere into one of the robots. They release the Yeti back to the Intelligence's control, saving their secret for when they need it. 

Jamie and Lethbridge-Stewart set out to rescue Victoria. On the way they find Arnold and return him to the HQ before continuing on to Piccadilly Circus. Evans takes care of the injured Arnold, but they soon have to abandon the HQ as web is seeping through the walls. Evans flees in terror into the tunnels and is captured by a Yeti. 

The Doctor and Anne meet up with Jamie, the Colonel and Arnold. Arnold manages to escape once more when the others are taken by the Yeti to the Intelligence's control centre. He finds Chorley wandering the tunnels. 

Travers and Victoria are brought before the Intelligence's pyramid. The Doctor and the others are also brought into the room. Jamie manages to break away and grabs the microphone that controls the Yeti. In the confusion the Doctor uses his device to immobilise the Yeti guard and tampers with the mind draining helmet. 

Jamie is recaptured and Chorley enters. He reveals that Arnold is the traitor who has been working against them. The Intelligence has been animating his dead body all the time. Arnold/the Intelligence forces the Doctor to don the helmet whilst Arnold's body wears another helmet and the process begins. Jamie uses the microphone to order the Yeti to attack and the Intelligence's pyramid is destroyed, breaking its contact with Arnold's now charred body and the Earth. The Doctor is angry with Jamie as he had altered the helmet to drain the mind of the Intelligence. The three travellers depart for Covent Garden before Chorley can interview them.

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