Professor 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.
What could he possibly want from the criminals? And what connects him with an impending World Peace Conference?
Special features include: commentary, behind-the-scenes interviews, a look back at the filming locations used in the story, a visit to the BBC Television Centre, photo gallery, programme subtitles, subtitle production notes, Radio Times Listings (PDF), 1971 Kellog's Sugar Smacks promotion (PDF).