I don't remember when I first happened to see The IPCRESS File, but I do know I thoroughly enjoyed it. The 1965 movie is an adaptation of a novel by the same name by Len Deighton and is espionage with a bit more reality and tooth than was perhaps found in the on-screen adventures of James Bond, Derek Flint and other super-spies. Harry Palmer is no "super-spy", that's for sure. As played by Michael Caine, Harry Palmer (a named derived apparently as sounding like the least impressive possible) is very much a near-sighted man with feet of clay who chases women and seems constantly to be trying to find ways to slip out of his responsibilities at work. He's a guy who was a minor criminal in the army who serves in MI-5 as opposed to going to jail. He is, to put it mildly, a small-time scoundrel.
The story of this movie concerns missing scientists who seem to be part of a foreign scheme to drain the smarts out of the West. Palmer gets involved in a small outfit looking into the latest disappearance and using some techniques not quite approved by his immediate superiors makes surprising headway. There is death and some small-time mayhem in this sometimes gritty adventure, but by and large we have a low-key smart little glimpse of spies and spying and the common dreariness which just might be a significant part of the whole shebang.
By the time of the third Michael Caine in 1967 the hard-bitten core of the debut was all but gone in a spy romp that abandons the minor tropes which made Palmer so compelling, even his signature glasses seem to go missing for extended periods of time. Billion Dollar Brain is a movie which could've been populated by pretty much any super spy of the era and for all its bombast falls flat fairly quickly. It makes more sense than its immediate predecessor but is much less satisfying as a film experience.
I always wondered why The IPCRESS File played TV so often (relatively speaking) while the sequels never did. Now I know, they sorta' suck.