It's All In The Mind!
If the events of the Prisoner are all in the mind, or partly, in Number 6's mind then the natural starting point is whatever pushed him over-the-edge into his current mental state. At what point in the series in the mental loop actually begins is, of course, still open to more debate. Some of the more obvious candidates are;
* The resignation itself, so that everything we see in the village is imaginary.
* At the end of, or at some point in 'Fall Out,' the confrontation with No.1, having faced himself, to what he had become.
* The return home, having apparently 'escaped.'
* In fact, any sufficiently significant or traumatic event within the series.