Professor Emil Keller has created a machine that can pacify even the most dangerous of criminals. But when the Doctor and Jo arrive at Stangmoor Prison for a demonstration, things start to go horribly wrong - especially when they discover that the Doctor's old enemy the Master is responsible for the machine.
Director: Timothy Combe
Writers: Don Houghton, Terrance Dicks
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Still without his dematerialisation circuit, the trapped "Doctor" (Jon Pertwee) and his assistant "Jo" (Katy Manning) are summoned to a prison for violent felons to investigate a mystery surrounding the rather menacing "Keller" machine. It's a sort of high-tech gadget that substi…
