On a distant, barren planet, Storm Mine 4 trawls across bleak deserts and through fierce duststorms in search of rare and valuable metals. On board the Sandminer is a small skeleton crew, who alternate between indulgent relaxation and skilled mining work. The mundane, day-to-day duties of the mine are attended to by a much larger complement of servile robots. This is a society that is dependent on robots for all areas of life, the people comforted by the knowledge that the strictest safeguards are built into each and every robot's programming. So when one of the Sandminer crew is murdered, suspicion falls on two new arrivals... The Doctor and Leela arrive on board, and are immediately accused of being the suspects. But the Doctor deduces that perhaps the killer is possibly not human. More deaths occur - can he persuade the remaining crew members that the killer may be a robot?