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.
Jon Pertwee , Katy Manning , Nicholas Courtney , Richard Franklin , John Levene , Roger Delgado , Pik-Sen Lim , Michael Sheard , Neil McCarthy , William Marlowe
BBC
