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.

• BBC
• Jon Pertwee
• Katy Manning
• Nicholas Courtney
• Richard Franklin
• John Levene
• Roger Delgado
• Pik-Sen Lim
• Raymond Westwell