On a distant, barren planet, Storm Mine 4 trawls across bleak deserts and through fierce duststorms in search of rare and valuable metals. Onboard the sandminer is a small skelton crew, who alternate between indulgent revalation and skilled mining work.

The mundane, day-to-day duties of the mine are attended to by a much larger complement of serville robots. This is a society that is depentant on robots for all areas of life, the people comforted by the knowledge of 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 the killer is possibly not human. More deaths occur - can he pursuade the remaining crew members that the killer may be a robot?

This story originally broadcast: 29 January-19 February 1977