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.

The Doctor and Jamie tell the Brigadier about the Cybermen but with Rutledge under Vaughn's control he is powerless to act.

With the entire world unconscious apart from the Doctor and his friends, the Cybermen are poised to invade and only Zoe, the Brigadier and a missile crew can stop them.

Scotland, the near future. Something is smashing oil rigs off the Scottish coast, and UNIT have been called in to investigate. The Brigadier summons the Doctor to Earth to help in the investigation, but it soon becomes clear some kind of alien presence is at work...

The Zygon traps the Doctor and Sarah Jane in a decompression chamber while Harry is taken to the aliens' underwater ship.

The Doctor and the Brigadier begin to realise who the Zygon imposters among them are while Sarah Jane discovers a secret tunnel to the underwater ship.

UNIT attempt to track down the Zygon ship while Broton prepares to have the Skarasen attack London.

The Doctor suspects Crayford brought something to Earth in his rocket, as he and Sarah find themselves hunted by UNIT.

The Doctor is captured by Styggron and left to die in the destruction of the fake village but Sarah escapes from the disorientation chamber to try and rescue him.

The Doctor and Sarah reach Earth and try to warn the research centre about the Kraal invasion before Styggron unleashes his virus.