The Doctor and Jo arrive at HM Stangmoor Prison to observe a demonstration of a new device for treating hardened criminals, the Keller Machine. As Professor Kettering (Simon Lack) carries out the process, the prisoner being treated, George Edward Barnham (Neil McCarthy), collapses. Shortly afterwards, another observer, Arthur Linwood (Clive Scott), is found dead - he seems to have been bitten and scratched and Doctor Roland Summers (Michael Sheard) reveals that his medical history shows that he was terrified of rats. The prison governor (Raymond Westwell) asks Kettering to give the machine a thorough test. 

The Brigadier (Nicholas Courtney) is organising the security for the first World Peace Conference and gives a separate job of safely transporting the Thunderbolt - a nuclear powered missile with a warhead full of nerve gas - to Yates (Richard Franklin). Captain Chin Lee (Pik Sen Lim) of the Chinese delegation complains to the Brigadier that important documents have been stolen from General Chen Teik's suite. Outside UNIT's temporary HQ, she burns the papers herself, apparently in a trance. 

At the prison, Kettering is checking the Keller machine when it becomes active. He imagines water and gasps for air before he dies. Doctor Summers later says that all the indications are that he drowned, something Kettering was terrified of. The Doctor wants the machine destroyed but the Governor needs Home Office permission to do this. The Doctor learns that the machine has been in operation for nearly a year and that when Professor Emil Keller installed it, he had a Chinese girl as his assistant. The Doctor is with the machine when a riot starts among the prisoners. The machine activates and the Doctor sees fire and flames. The machine stops its attack when Jo enters the room. The Doctor realises he must find a way to control the machine. 

The Doctor is urgently summoned back to UNIT HQ and he leaves orders that no one is to go near the machine. The Doctor learns from the Brigadier that the Chinese delegate has been killed and when Benton (John Levene) tries to follow Chin Lee, she knocks him out with telepathic power from the Keller machine. 

Outside UNIT's HQ, the Master (Roger Delgado) bugs the telephones and listens as Yates outlines the route and plans for the transport of the missile. He then summons Chin Lee and orders her to kill the American delegate that night. 

The Brigadier and the Doctor visit the new Chinese delegate, Mr Fu Peng (Kristopher Kum), where the Doctor's knowledge of Hokkien stands him in good stead. When the Brigadier mentions a Chinese girl, the Doctor makes the connection with the Keller machine. 

That night, Chin Lee asks the American delegate, Senator Allcott (Tommy Duggan), to come to the Chinese delegate's suite. When he arrives she attacks him in the form of a Chinese dragon. The Doctor and the Brigadier arrive at the suite in time to stop her from killing the delegate and she reverts to human form and collapses. She has a telepathic amplifier behind her ear. 

At the prison, two prisoners, Harry Mailer (William Marlowe) and Lenny Vosper (Haydn Jones), take two guards hostage. They assume control of B wing and demand to speak to the Governor. The guards manage, with some help from Jo, to recapture Mailer, but the Master arrives in the guise of Professor Keller, and asks to see Mailer. He helps him to escape once more, and this time Mailer gains control of the prison. 

The Doctor arrives at the prison and discovers the Master in charge. The Master wants to hijack the missile, wipe out the peace conference and initiate a World War. He wants the Doctor to control the Keller machine and straps him to a chair beside it as he is curious to see how long the Doctor can stand up to it. With the telepathic amplifier placed on the Doctor's head, he sees images of his past enemies flash before his eyes before he collapses, along with everyone else in the prison. The Doctor realises that inside the machine is a creature that feeds on the evil in men's minds. The Doctor is put in a cell to recover and the Master is attacked by the machine - he sees the Doctor laughing at him. He runs from the room to escape. 

The missile convoy is attacked by freed prisoners as it passes close to the prison. The missile is hijacked and Yates is captured after he follows the missile to an abandoned airfield. The Brigadier tries to locate the missile by helicopter but fails. 

The Keller machine now has sufficient power to move about. It attacks anyone it finds, leaving them dead. The Doctor and Jo escape from their cell and encounter Mailer and Vosper. Vosper is killed by the machine and Mailer runs off, leaving the machine to attack the Doctor and Jo. The machine vanishes again as Barnham enters the room. All evil has been removed from his mind and he is as innocent as a small child. 

The Doctor is recaptured by the Master and builds a coil device to throw over the machine and immobilise it. This works and the Doctor and Jo are again placed in a cell. 

UNIT troops attack the prison and the Brigadier shoots Mailer before he can kill the Doctor. The Master has returned to where the missile is hidden and aims it. Yates, who has escaped, calls in from a mobile HQ with the location - Stanham, a deserted airfield. The Brigadier returns to the mobile HQ leaving the Doctor to try and deal with the machine. 

The Doctor and Jo discover that Barnham's presence inhibits the machine - he has no evil for it to feed off. The Master contacts the Doctor by telephone. He wants to deal: his dematerialisation circuit for the missile. The Doctor agrees to meet him at the airfield. 

The Master has disabled the abort mechanism on the missile so that when the Brigadier tries to destroy it using this option, nothing happens. 

The Doctor realises that he can use the Keller machine against the Master and so arrives at the airfield in a prison van. With the Master distracted, Jo and Barnham, who were hiding in the back of the van, put the machine down beside him and run off. The machine immediately starts to attack the Master. The Doctor re-connects the abort mechanism and then runs for the waiting UNIT helicopter with Jo and Barnham. Barnham stops to help the Master, and as the machine's attack fades, the Master leaps in the prison van and runs Barnham down. The Brigadier blows up the missile, and the Keller machine is caught in the blast and hopefully destroyed. 

Back at the prison, the Doctor takes a telephone call from the Master who is alive and well and has his dematerialisation circuit. He promises to destroy the Earth one day.

_{Synopsis from Doctor Who: The Third Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}