Earth, the near future. The Doctor visits Stangmoor Prison to see the Keller Machine, a revolutionary new type of rehabilitation aid, in action - but finds his fellow observers literally dying of fear. Who is responsible for creating the Machine? And how does it connect to the peace conference happening in London?
Elenco: William Hartnell , Patrick Troughton , Jon Pertwee , Tom Baker , Peter Davison , Colin Baker , Sylvester McCoy .