As the episode begins, Number 6 is assisting Number 24, a telepathic young woman, in practising mind reading with Zener cards. As an extremely complex plot of bluff and double bluff ensues, Number 2 brings a lookalike of Number 6, referred to as "Number 12", to The Village. Number 12 (also played by McGoohan, apart from a few shots with a double) is not a clone, but an "agent" of The Village who happens to bear a very strong resemblance to Number 6. Details in the final scenes of the episode reveal Number 12 to have a wife outside The Village.
The real Number 6 is subjected to an intensive course of aversion therapy, altering his tastes and instincts, and training him to do everything left-handed. He is drugged to wipe his memory of the treatment. When he awakes, he is treated as "Number 12", while the lookalike assumes the role of Number 6. The real Number 6 is informed by Number 2 of the plan to break "Number 6" (the impostor) by convincing him that he is not Number 6 at all.
6 and 12 engage in various challenges to prove which is the real Number 6, the aversion therapy resulting in the impostor behaving more like Number 6 than the real one does. In the presence of Number 2 and Number 6, Number 12 is challenged to demonstrate that his fingerprints are Number 6's. They are. He also has his characteristic left wrist mole, which Number 6 has lost. Finally, Number 24 is summoned because she has a unique "mental bond" with the real Number 6, but Number 6 fails a test with the Zener cards.
Just as he appears to be "breaking", the real Number 6 mentally overcomes his brainwashing when he discovers a bruise on his fingernail that he got when Number 24 tried to get his picture – a bruise that, furthermore, has migrated from the base of his fingernail to midway, confirming that days or weeks have passed, not the single day shown on his calendar. He then gives himself an electric shock to reverse the therapy. He also physically overcomes the impostor, who reveals his name as Curtis, after making him reveal his password to identify with Number 2 and removing the fake mole from his wrist, Curtis escapes and is then mistakenly killed by Rover. Pretending to be Curtis, Number 6 reports to Number 2 that "Number 6 is dead." Having "failed", he is to return to report failure. He is put blindfolded onto the helicopter to leave The Village. He believes himself to have duped Number 2 into letting him escape, but the helicopter promptly returns to The Village. Number 2 reveals that he had deduced the truth when Number 6 agreed to give his regards to Number 12's wife, who is deceased.
The character of Alison (Number 24) is unusual in that she is a Village resident/inmate yet is referred to by name not only by Number 6 but by Number 2. She is also one of the only female characters in the series to have any sort of relationship with Number Six in which Number Six reciprocates (their relationship is seen purely as friendship, not romance).
This is the only episode in which Rover is referred to by name. Prior to this, the device was never referred to by any name, although scripts and the spin-off novels refer to it as the Guardian. It is also the only time where Rover seemingly kills somebody, rather than merely incapacitating them.