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.