Professor Travers of the Tibetan expedition seen in The Abominable Snowmen tries to get the owner of a museum to return the Yeti robot. Professor Travers claims to have reactivated a control sphere, and now it has gone missing. Silverstein, the owner, throws Travers out. The control sphere activates the Yeti, which kills Silverstein.
Jamie, the Doctor, and Victoria, land after some trouble—the TARDIS is frozen in space—in the Covent Garden London Underground railway. The station is abandoned, and it is broad daylight outside. Jamie spots an old man seated outside the gate, but they find he is dead and covered in cobwebs.
Professor Travers is brought by army troops to a WWII shelter adjacent to Goodge Street tube station. He starts off by annoying Harold Chorley, the only journalist allowed there during the problem that is spoken of constantly. The fortress commander, Captain Knight, disapproves of Travers' presence.
Below ground the trio see three soldiers unwinding a drum of cable along the tunnel. Jamie and Victoria are discovered and captured.
The Doctor follows the cable as far as Charing Cross underground station. He hides when he hears a pair of Yeti coming. The robots cover some crates with a thick cobweb substance.
Jamie and Victoria are taken to the Goodge street fortress. They learn that the tunnel is going to be blown up.
After the Yeti have left the platform the Doctor goes to the crates to examine them and is caught by a contained blast.
The soldiers soon work out that the blast in the tunnel did not register in the normal way. Jamie and Victoria find out about the Yeti and contact Travers. He is overjoyed to be reunited with his old friends from Tibet. Knight, Corporal Lane and some other soldiers find out that an ammunition party with a truck at Holborn tube station have all been killed.
Jamie heads off with Staff-Sergeant Arnold to see if they can find the Doctor. They soon meet up with Knight and Lane, who have come under attack. When they try to detonate some explosives against the Yeti, the bombs fail to explode. The Yeti retreat. In the underground there is poison web fungus expanding in the tunnels. It seems to be engulfing the whole of the Circle Line.