The Doctor and Peri arrive on Androzani Minor in a vast underground cave system, where they are mistaken for gun-runners by General Chellak's soldiers. Sentenced to death, they are rescued by Sharaz Jek, a hideously disfigured ex-scientist who is plotting his revenge on Morgus, the corrupt industrialist who caused Jek's disfigurement. The Doctor and Peri find they are both suffering from Spectrox Toxoemia, a progressively fatal condition that can only be cured by the milk of a rare bat. As the effects of the Toxoemia become worse, can the Doctor find the antidote before they both die?