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.
Writer: Don Houghton
Director: Timothy Combe
Producer: Barry Letts
The Doctor
Jo Grant
Brigadier Lethbridge-Stewart
Captain Mike Yates
Sergeant Benton
The Master
Captain Chin Lee
Prison Governor
Mind of Evil | Doctor Who DVD