The filming of this episode commenced on December 19, 1966.

The name of the guardian sphere, Rover, is revealed in this episode. Interestingly, it was first used by Number Six (Patrick McGoohan) and is never used in any other episode.

Number Six: (to Number Two) It has fascinating possibilties but you'll have an awful lot of trouble convincing I am not your Number Six.

Number Twelve: Where did they get you, a people's copying service or are you of those double agents we hear so much about these days?

Number Twelve: (to Number Six) I take it I'm supposed to go all fuzzy round the edges and run off into the distance screaming "Who am I?"

Number Twelve: (to Number Six) Do you know, I never realised I had a freckle on the side of my nose...when they come to film my life story, you've got the part.

Number Twelve: You are the goodie Number Six and I am the baddie who is supposed to be proving you wrong, is that it?

Number Twelve: You know what, why don’t we settle this like gentlemen?
Number Six: You’re claiming to be a gentleman too?
Number Twelve: Oh very good, very good indeed. That line is very worthy of me.

Number Twelve: (to Number Six) If you ever do challenge me to a duel, your safest bet would be battle axes in a very dark cellar.

Number Twelve: (about Rover) Oh, dear. Looks like we're in trouble with the headmaster.

Number Twelve: I am Number Six! I am Number Six! Number Six! Six! Six! Six! Six! SIX!

Number Two: By the time we finish with him, he won't know whether he's Number Six or the cube root of infinity.

Number Twelve: The trouble with science is that it can be perverted.

Number Six: (to Alison; about Number Twelve) I am the original, he is the economy pack.

Number Six: Number Six is dead.
Number Two: What?
Number Six: He’s dead. Rover got him.

Torture and Escape
While the torture of "Free for All" is general, the torture here is more individually tailored as the Village authorities attempt to strip Number Six of his very identity by convincing him he's someone else. Unlike "Free for All," which brainwashes Number Six into going along, here he is simply conditioned to be left-handed and like flapjacks. This makes his near-breakdown much more convincing. The escape attempt at the end is added on almost half-heartedly and it's clear that it has no chance of success. Still, this is another case where Number Six earns a hard-won victory.