The Caves of Androzani — Terrance Dicks

From the moment they land on the planet Androzani Minor, everything goes wrong for the Doctor and his new young companion, Peri.

They become involved in the struggle between brutal gun-runners, ruthless Federation troops, and the hideously mutilated Sharaz Jek, who lurks in the depths of the caves with his android army.

Key to the struggle is spectrox, the most valuable substance in the universe. Suitably processed, spectrox is an elixir of life, but in its raw state it is a deadly poison--a fact which will cost the Doctor another of his Time Lord lives...