Fantastic Voyage (1966), directed by Richard Fleischer.
In a very 1960s secret underground base, a disparate team are shrunk to microbe size in a mission to repair a brain clot in a scientist -- from the inside. Secret agent Stephen Boyd is along to catch the saboteur.
It's still pretty wonderful, the sort of adventure every kid dreams about. The very cool Proteus submarine is by Harper Goff, who did the Nautilus for 20000 Leagues Under the Sea (1954). According to the commentary track he did other futuristic vehicles in 1960s SF -- the Jupiter II from Lost in Space and the spacecraft in Planet of the Apes (1968) -- but I can't find any credits for those.
It's good that they are briefly matter-of-fact about the miniaturization wizardry. Why flog such an unbelievable idea?
The physics behind this doesn't work at all, but that would be ok if it weren't for a gaping plot hole. Everyone with more than two brain cells begged the producers to revise the script, but the execs (I imagine them with big cigars and pinky rings) waved away the objections: "No one will notice". And yet, small children left the theater asking: "Why didn't they get the submarine out of the body?"
Strong supporting cast: Edmond O'Brien, Donald Pleasence, Arthur O'Connell, Arthur Kennedy.
First prominent role for Raquel Welch, although One Million Years B.C. (1966) is usually listed as her breakout. A story from the commentary track: in the scene where she is crushed by the antibodies, all the men have to jump on her and tear them off. They were gentlemanly about it, but that left her with an antibody brassiere. Take 2: they enthusiastically groped her boobs. Take 3, split the difference... and print it!
After much arm-twisting Isaac Asimov wrote a novelization of the screenplay. I read it when I was young and remember it as a better paced SF adventure than the movie. He levered in as much plausibility as he could, even getting the submarine out of the body, I think.
Available on Blu-ray. Good quality, although that reveals flaws in the process shots, which have a lot of errors by today's standards. Informative commentary track.
The first 40 minutes -- per the composer's insistence -- has no music, and this interval is used in an isolated score track for an enthusiastic appreciation of Leonard Rosenman, who they think has been overlooked as a film composer. With Alex North, Jerry Goldsmith and Elmer Bernstein he brought more modern music into what had been classic-based Hollywood. Credit is also due to Lionel Newman, music director at Fox, for mentoring these composers.