The TARDIS becomes trapped in space before ending up in an underground station of a mysteriously deserted London.

Jamie and Victoria are reunited with Travers before Jamie joins the soldiers in attempting to find out what has happened to the Doctor.

The Doctor and Victoria are taken back to the Goodge Street base by Colonel Lethbridge-Stewart where the Doctor becomes convinced there is a traitor in their midst.

With Travers having been captured by the Yeti, the Doctor attempts to find a way to gain control of the robots while Lethbridge-Stewart tries to recover the TARDIS.

Victoria and Travers are held prisoner by the Great Intelligence, who is planning to drain the Doctor's mind.

The Doctor and his friends are held prisoner by the Intelligence and their only hope lies with Jamie, Arnold and a reprogrammed Yeti.