Number Six (Patrick McGoohan) is assisting Number Twenty-four, named Alison (Jane Merrows), who is telepathic. They are using Zener Cards, which assist in mind-reading. Number Two (Anton Rodgers) brings in an agent who looks exactly like Number Six and is called Number Twelve, as he is the double of Number Six. The real Number Six is given intense aversion therapy in his sleep and ends up doing everything left-handed. Everyone refers to him as Twelve. Eventually, Six is convinced that he is Twelve and his job is to prevent Number Six from escaping.

They engage in various challenges to prove this. Twelve has Six’s fingerprints, and he has a mole on his left wrist, which Six no longer does. Number twenty-four is summoned because she has a psychic link with Six, but now she has it with Twelve and not at all with Six. Just as Six is about to believe he is Twelve, he notices a bruised fingernail that he had gotten many days before. It has travelled up the nail as the nail grew. More time has passed than thought. He gives himself electroshocks to reverse the effects of his aversion therapy.

He overpowers Twelve, who reveals that his name is Curtis. Curtis runs away from the Village and is killed by Rover. Now pretending to be Curtis, Number Six reports the death to Number Two. Just before he is put aboard the helicopter to be taken away, Number Two asks him to give his regards to his wife. He says he will. The helicopter circles back again and he is in the Village once more. Number Two points out that Curtis’ wife is actually dead.

The episode was directed by Pat Jackson and written by Terence Feely, who also wrote for The Saint, UFO, and the Avengers series. Alison is one of the few characters with a name. An actor named Earl Cameron played the Village supervisor, practically the only black in the series and certainly the only one with authority. McGoohan’s stunt double, Frank Maher, played Number Twelve at various times. Patrick McGoohan was supposed to play a character called Schizoid Man in an episode of Star Trek: The Next Generation, which ended up not happening, but they kept the episode title. Number Twelve’s jacket was black trim on white and Six’s white trim on black, but McGoohan was such a good actor that viewers eventually knew who was who anyway.

This is the only episode in which Rover is mentioned by name. According to production manager Bernie Williams, they had a hard time creating it in the first place. They needed something that could pursue targets in the streets, in the countryside, on the beach, and in the water. At first, they tried a little armored vehicle, driven by Bernie himself, who could not see where he was going and nearly choked on the engine fumes. It was laughable on screen. So Bernie and Patrick sat down outside a bar with gin-and-tonics and hashed out ideas. Finally, someone looked up and saw a weather balloon in the sky. They found that, filled with helium and pulled by wires, it could rise out of the sea, bounce along as if pursuing runners, and flatten on their faces in a creepy manor as if asphyxiating them. It turned out to be a favorite character in the show.