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.

Science Fiction, Drama, Adventure

Jon Pertwee, Katy Manning, Nicholas Courtney, Richard Franklin, John Levene, Roger Delgado