The Doctor and Peri arrive on the planet of Androzani Minor and upon investigating some nearby caves they brush against a strange web-like substance. Soon afterwards they are arrested by the army of the neighbouring Androzani Major on suspicion of being gun-runners for the rebel known as Sharez Jek who has holed himself up in the caves with an army of androids having stolen the only supply of a drug called Spectrox, said to prolong life.

Rescued by Sharez Jek, who has fallen in love with Peri, they discover that all he wants is revenge against Morgus, a wealthy and corrupt businessman who was resposible for horribly disfiguring him.

The Doctor and Peri soon realise that the substance they touched, which is killing them, is raw Spectrox and they must quickly find an antidote. However, on a world of android doubles and double-crosses this is easier said than done as Jek and Morgus battle it out for vengeance and the control of Spectrox.