The Doctor and Jo visit Stangmoor Prison for a demonstration of the Keller Machine - a device claimed to be capable of extracting negative emotions from hardened criminals.

The Doctor's scepticism seems valid when a prisoner called Barnham collapses whilst undergoing the treatment.

The Brigadier is meanwhile in charge of security at a World Peace Conference, where documents go missing and the Chinese delegate dies in mysterious circumstances. Captain Yates is away on another mission, transporting a banned Thunderbolt missile across country to be destroyed. The Doctor joins the Brigadier at the conference and they foil an attempt by the Chinese delegate's aide, Captain Chin Lee, to kill the American delegate. Lee is under the hypnotic control of the Master - otherwise known as Professor Emil Keller.

The Master uses the evil impulses stored within the Keller Machine - actually the container for an alien mind parasite - to cause unrest at Stangmoor. He then enlists the convicts' aid to hijack the Thunderbolt missile, planning to use it to blow up the peace conference and start World War Three.

Shielded by Barnham, now immune to the effects of the parasite, the Doctor transports the Keller Machine to a nearby airfield where the missile is being held.

Using the Machine to keep the Master occupied, he reconnects the missile's auto-destruct circuit and gets clear just before the Brigadier triggers it. The parasite is presumed destroyed in the resulting explosion, but the Master escapes in a van, running Barnham down in the process.