Liz offers the Doctor a spare ticket to a conference in Paris on psi powers and psychic phenomenon; Professor Brockhurst, who was originally to attend the conference, recently choked to death in a restaurant. Their plane's engine unexpectedly explodes, but a telepathic young woman on the flight is able to keep it in flight. The Doctor keeps her mind focussed on her task until the plane lands, although this costs the young woman her life. Liz recognizes her as a student of Professor Hardin, Brockhurst's rival at Cambridge, but when they return to Cambridge to confront Hardin he drives them out of his office without explaining his work. Suspecting that Hardin is responsible for Brockhurst's death and is conducting unethical experiments upon student volunteers, the Doctor calls in the Brigadier and with Liz's help organises a mass protest against Hardin. Hardin has been conducting surgery on his students to release their psychic potential, and apparently has been using a psychokinetic student to kill his rivals; the plane was brought down to prevent the young woman from revealing Hardin's unethical experiments to the conference. The Doctor realizes that Hardin is trying to create an army of psychic assassins to sell to the highest bidder, but when he confronts Hardin he discovers that Hardin has also conducted experiments upon himself. Having killed his unstable assassin, Hardin attempts to crush the Doctor's lungs within his body, but at that moment the Brigadier enters and shoots him dead.

The Doctor and Jo are called away from a scientific conference by an urgent message from the Brigadier, and arrive at a UNIT training area to find Colonel Ashe waiting for them. Ashe claims that the Brigadier has been unavoidably detained and that they must meet elsewhere, but as they approach Ashe's helicopter Ashe erroneously identifies a target Auton as an Ogron -- revealing that he isn't a UNIT soldier after all. The Doctor gets away but Ashe threatens to kill Jo if the Doctor doesn't show himself; Ashe is in fact a Soviet officer who has come to take the Doctor to the other side of the Iron Curtain to work for them. The Brigadier arrives just in time to rescue the Doctor and Jo, or so he thinks until the Doctor reveals that while Ashe was holding Jo prisoner, the Doctor was filling the gas tank of Ashe's helicopter with sugar.

The Third Doctor visits UNIT's detention centre at Aylesbury Grange, where the Master is being held prior to his trial. The Master claims that he has reformed and wishes to help repair the damage he has caused over the years, but much as the Doctor would like to believe him, he can't. The Master then reveals that he's already escaped and disguised himself as his own guard; the Doctor has been speaking to a holographic illusion. But as the Master departs he is confronted outside the cell block by UNIT troops and by the real Doctor. Suspecting the Master's intentions, the Doctor had sent in a hologram to speak with the Master.