As some other reviewers have opined, this is the best of the three Harry Palmer movies set in the '60s. Michael Caine's cavalier attitude coupled with his witty, sarcastic banter is most refreshing. All the stodgy bureaucratic types get bum-rushed by Harry Palmer's rapier tongue. His self deprecating humor, especially in some of the scenes with Samantha Steele (Eva Renzi), is refreshing, since she has the pertness and sense of humor to complement those scenes deftly. Similarly, the scenes with Col. Stock show great flashes of sarcastic wit, juxtaposed with attending to the serious Cold War business at hand.
But the more serious fun is the wonderfully executed plot development, interweaving the various characters such as Johnny Vulcan, Col. Stock, Hallam, Ross (the boss), Kreutsmann, Steele, etc. into a menagerie of complicated intrigue. I'll let others offer a plot synopsis: I'll just say the film keeps you guessing and wondering throughout its hour and forty-five minutes. The overall style of mixing mordant anti-establishment humor and complicated intrigue reminds me of a later film, "The Russia House", with Sean Connery, an equally satisfying Cold War drama.
The only knock I have on this film is the sometimes obtrusive sound track, where the trumpets blare much too loudly to proclaim a given dramatic occurrence. Could have been done a little more tastefully.
All in all, a "must see" for Cold War movie fans!