Michael Caine's Harry Palmer -- the character is nameless in the Len Deighton novels; as he is also the first-person narrator, this works, but for this film, (third-person all the way) it was felt that he needed a name -- is just as escapist a fantasy as Connery's Bond, but in a different manner. Deliberately deglamorised and *presented* as just a relatively ordinary man, if of a somewhat dubious moral character, doing his best to keep out of trouble, Palmer nonetheless is, underneath, a bit more.
Blackmailed into espionage with the threat of well-earned prison time, Palmer is a useful foot-soldier in the sordid, quiet war of espionage and counter-espionage, set to unmask a traitor -- but who *is* the traitor -- is there anyone at all that he can trust?
Michael Caine (this was the first film in which i had seen him) inhabits the role of Harry Palmer and makes it totally his, a man of contradictions -- a working class man, but one who genuinely loves and appreciates the finer things, unlike Fleming's (and, to some extent, the Bond movies') Bond, an amoral thug who apes the manners and tastes of his betters.
The apparently-realistic dreary grey London streets and settings add to this film's apparently-realistic approach, all the better to persuade the viewer to suspend his disbelief and accept the rather complex plot, especially when we get to the brainwashing parts...
First of three films, this was a series that *could* have rivalled Bond but fizzled out in the end.
All three, however, are well worth your time.