Henry Phyfe is an agent with the CIS.
That acronym stands for the Central Intelligence Service. That is, apparently, another piece of the U.S. alphabet intelligence community. In this agency, one of the men near the top, though not the head man, is Gerald Hannahan, referred to as the Chief, a man in his early 50s who acts as annoyed and ill-tempered as he looks. Mostly bald with a mustache, he is as quick with a smile that he obviously does not feel as he is with a frown which he does.
Phyfe, as an operative with the CIS, acts under the codename of U-31.
Except that he isn't and he doesn't.
The following opening lines from the televised adventures give the basis for that dichotomy (Note: the speaker of the parts in quotes is Chief Hannahan):
A foreign spy arrives by the name of U-31.
The first day in he's done in by a hit-and-run.
'Gotta find a man with the same face as U-31, but who?'
He swears him in and gives all the info to Henry Phyfe.
He must never talk to a soul about his secret life.
'You are now the spy. You must now lead a double life.'
'Who me?' ... 'Henry Phyfe!'
If he could only keep his disguise a big mystery,
This could become the greatest deception in history.
'With a little luck he might even fool the enemy.'
So the concept is that a notorious enemy agent arrives in the States on some sort of mission, undoubtedly nefarious, and is promptly killed (accidentally?) in a car-pedestrian accident. Chief Hannahan realizes the incredible opportunity handed the CIS if they could find someone who looked enough like the dead spy to take his place.
As 'un-luck' would have it, they did in the form of Henry Phyfe, as mild-mannered and timid and meek a man as one could hope/fear to find. He is a dead-ringer (no pun intended) for the former U-31. Unfortunately, he is as far from being a skilled operative as one could get.
What he is in real life is an accountant for the firm of Hamble and Hamble, having to deal with demands of an overbearing boss, one of the Hamble's in the name. He has a girl friend named Judy who is sweet and caring and a good match for Henry. Unfortunately she has a mother named Florence who is also quite overbearing and not at all believing Phyfe is good enough for her daughter.
When you consider the three facts that plague Phyfe in this matter - a) that Hannahan will often come calling upon Phyfe for 'one more mission' and b) he maintains the demand that Phyfe never tell anyone (read his boss or his girl friend or his girl friend's mother) what he is doing and c) he (Phyfe) is truly no good at it whatsoever - you can understand why Phyfe is frequently asking the question, "Who, me?!"
When told yet again this next assignment would be the last one, Phyfe responds, "Mr. Hannahan, you always say I'll never see you again but somehow I'm always the only name on your dance card."