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.

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

Genre:Science Fiction, Drama, Adventure,

country:United Kingdom,

Production:BBC,