"Once Upon a Time" is the penultimate episode of the allegorical British science fiction TV series, The Prisoner.

It was first broadcast in the UK on ITV (Scottish Television) on Thursday 25 January 1968 (it appeared on ATV Midlands and Grampian the day after) and first aired in the United States on CBS on Saturday 14 September 1968.

The Number Two from the earlier episode "The Chimes of Big Ben" (Leo McKern) returns to the Village.

Number Six is put into a trance state, causing his mind to regress back to his childhood.

Suddenly, Number Six starts counting down from "six", and by the time he has reached zero, has regained full control of his mind.

Now in control, Number Six explains that Degree Absolute, a well-known psychiatric technique, has its risks to the one performing the therapy if they have their own psychological problems.

According to The Prisoner: The Official Companion to the Classic TV Series by Robert Fairclough, the strain of filming this episode caused McKern to suffer either a nervous breakdown or a heart attack (accounts differ), forcing production to stop for a time.

John Maxim's brief scene as Number Eighty-Six was cut from the episode, however he is still listed in the credits.

[1] This episode is referred to by Number Two in the 1988 comic book sequel Shattered Visage, when asked by Alice Drake whom Six killed.