With the newly-regenerated Doctor (Tom Baker) behaving erratically, Brigadier Lethbridge-Stewart and UNIT have to contend with the theft of the plans for the new disintegrator gun by what seems to be a mechanical monster. However, the theft is only the beginning of a plot to hold the world to nuclear ransom.
The TARDIS arrives on an apparently deserted and deactivated space station Nerva, otherwise known as the Ark, orbiting Earth in the far future. There the Doctor, Sarah and Harry discover the last survivors of the human race held in suspended animation, Earth having been evacuated thousands of years earlier when solar flares threatened to destroy all life. The station has been visited by Wirrn, an insect life form, which has laid its eggs in the solar stacks and absorbed the mind and body of one of the sleeping humans. The Doctor’s reactivation of the station’s systems causes the humans to start to revive
Arriving on a desolate and windswept Earth the Doctor starts realigning the transmat refractors while Sarah and Harry go off to explore. Harry slips and falls into a pit and Sarah runs back to the Doctor to get help. The Doctor, however, has been captured by a trio of shipwrecked Galsec space travellers who were lured to Earth by a phoney distress call. Sarah, unable to find the Doctor, makes friends with a fourth space traveller, Roth, who tells her of the alien in the rocks who is experimenting on them…
The Doctor and his companions are transported back in time by the Time Lords who have decided that the Daleks need to be destroyed. The Doctor’s mission is to change history so that they have never existed. Soon they are separated and The Doctor and Harry are captured by the evil Elite of the mad scientist Davros, who is working to perfect his Mark 3 travel machine: the Dalek.
The time ring takes the Doctor, Sarah and Harry back to Nerva, but to a period many thousands of years earlier than their previous visit. This is Voga, also known as the planet of gold as that metal can be found in abundance there. The three friends learn that a space plague has killed all but a handful of Nerva’s crew. A visiting civilian scientist named Kellman is in fact a traitor working with a group of Cybermen who want to destroy Voga as gold dust can coat their breathing apparatus and suffocate them. Can the Doctor stop them?