The Doctor arrives on Space Station Nerva in its distant past to find its crew threatened by a mysterious plague. Discovering that things are not as they seem, the Doctor stumbles upon a Cybermen created plan to commit genocide.

The Doctor and Adric learn from the wizened Keeper of Traken that a great evil has come to his planet in the form of a Melkur - a calcified statue. The Keeper is nearing the end of his reign and seeks the Doctor's help in preventing the evil from taking control of the bioelectronic Source that is the keystone of the Traken Union's civilisation.

The Doctor lands on Chloris.

The Doctor is trapped in a pit and must confront the resident monster.

The Doctor makes a final stand against an army of Cybermen, to protect a tiny band of humans from destruction.

In a South Yorkshire city, Ryan Sinclair, Yasmin Khan, and Graham O'Brien are about to have their lives changed forever as a mysterious woman, unable to remember her own name, falls from the night sky.