
The recap below contains plot spoilers about Episode 13, "Do Not Forsake Me Oh My Darling." If you haven't seen "Do Not Forsake Me Oh My Darling" yet, you can watch the full episode online.
No. 6 undergoes an eerie transformation that transmits his mind and personality into another man's body.
No. 6 (Patrick McGoohan) is one of the few men who might know how to trace the missing Professor Seltzman (Hugo Schuster): He was the last man to have contact with him before Seltzman vanished and before his own resignation.
And now No. 2 (Clifford Evans) has the task of tracing Seltzman through No. 6.
Seltzman is the inventor of a process that transmits the mind and personality of one man into the body of another -- and this process is used on No. 6, who awakens to find himself in his own London home but a stranger to himself when he looks into a mirror and find that he has the body of an Army colonel (Nigel Stock).
His fiancée Janet (Zena Walker), fails to recognize him, of course. So does her father, Sir Charles Portland (John Wentworth), one of the No. 6's former superiors who had been responsible for putting him on the Seltzman case.
The important thing for No. 6 is to trace the man he has been "reversed" with, but in the meanwhile he attends Janet's birthday party, where he convinces her that he knows her fiancée sufficiently well for her to pass over a receipt that he had left with her for safe keeping: a receipt for a bundle of color transparencies.
Sir Charles and his men already have copies of these photographs, but are unable to make any kind of sense from them. No. 6, however, puts them into order and, with the aid of a special code, discovers the name of a place where Seltzman can be found.
He goes there, posing as a Village barber, and tells Seltzman what has happened. Seltzman tells him that, although he perfected his process, reversal is extremely tricky. While they are still talking, the house is broken in to, and the two men become victims of a nerve gas. They return to consciousness in The Village, where No. 2 rejoices in the fact that he is responsible for the capture of Seltzman. But can No. 6 and the Colonel be changed back into themselves? The attempt to do so has unexpected results.