Doctor who: the caves of androzani (1984)

Arriving on the barren world of Androzani Minor, the Doctor and Peri find themselves embroiled in a long running, literal underground war. At the heart of the conflict is a substance called Spectrox - both valuable and deadly! The Doctor & Peri wind up being poisoned by the material, which is killing them slowly and painfully unless they can find a cure. As the conflict heats up and the situation gets more desperate, the Doctor realises time is running out - both for Peri and himself...