Parts: 6
Writer: Don Houghton
Airdates: 30 January 1971 - 6 March 1971
Production Code: EEE
Megalomaniac of the Week: the Master
Special Guests: UNIT (the Brigadier, Benton, Yates)
Story: The Doctor takes Jo to Stangmoor Prison to witness the demonstration of a wondrous device. The Keller Machine, as it is called, has been tested hundreds of times and been proven to extract the violent impulses from the minds of criminals, leaving them able to be productive members of society. But the Doctor is unconvinced that the Keller Machine is as innocuous as claimed - for example, what happens to all the violent impulses?
Soon mysterious deaths begin occurring in the room housing the Keller Machine. A medical student is found, killed by a heart attack, with his face covered in small scratches and bites. Medical records show that he had been terrified of rats...but there are no rats in the prison. Then a technician is found with his lungs full of water, as though he had drowned in a completely dry room. Before long, the Doctor too finds himself in danger of facing one of his worst fears - being burned alive...
The person behind the invention of the Keller Machine is none other than the Master, who naturally plans to use the chaos to his advantage. The Master wishes to use the prisoners to help him hijack a powerful missile, with which he can destroy a peace conference and plunge Earth into a third World War. But even the Master does not realize the power of the Keller Machine, even he cannot control it, and it is eager to unleash itself upon the world...