The TARDIS, carrying the Doctor and his new companion Leela, arrives aboard a huge sandminer on a deserted world. The small human crew relies almost entirely on robots to carry out their every task and whim while they mine the planet's rich minerals. Suddenly, one by one, members of the team start to disappear. The time travelers discover that someone or something is murdering the crew - but, of course, nobody believes it could be the robots.