The TARDIS materialises inside an intake duct on board the Sandminer, a mining and refinery vehicle. With no obvious way out the Doctor and Leela are almost torn to pieces during a sandstorm, but the duct is closed just in time.
Returning to the TARDIS they find it has been moved to another part of the vehicle. The Doctor and Leela are captured and accused of the murders of crew members.
Why is it that every body that has been found has a red corpse marker on it? What does it symbolise? The Doctor discovers that the robots that serve the crew are slowly being reporgrammed to kill, and one of the non-speaking Dumb robots, D84, is actually a spy and can speak.
The Doctor manages to uncover the person who is in charge of the reprogramming, and is saved from death by D84. The Doctor hides Leela in a ventilation duct with a canister of helium. As she lets the gas escape the man voice of the man doing who had been doing the reprogramming, Taren Capel, changes pitch. The robots, not recognising his voice print, turn on him.
