Fantastic Voyage (1966)
A defecting scientist from behind the Iron Curtain is mortally wounded while trying to flee to the West and it's up to a American intelligence agent Grant to keep him alive during the brain surgery needed to save his life so he can pass on information vital to the survival of the Free World. It's no ordinary surgery because it involves having the operating team, including Grant, placed in a submarine and then shrunk to the size of a microbe before injecting them into the stricken scientists body. Unfortunately, there's an assassin among the team.
Oh, and the process only works for 60 minutes, so they have to work very fast. That doesn't help matters when a string of accidents sends the tiny submarine off course and the miniature medicos must take an unscheduled tour of the human body.
Often copied and usually blatantly ripped off, Fantastic Voyage was one of those "event" films that kept cropping up in the 1960s. It was lavish, boasted a budget that put every penny on the screen and had it been much longer (100 minutes; a long time in those days), they probably would have added an intermission. The plot is a pretty simple one, the team must find a way to the injured section of the brain while fighting such menaces as driving through a beating heart, replenishing lost air via the lungs and battling hostile white blood cells. Meanwhile, Grant must discover who the assassin is who keeps arranging "accidents" and will stop at nothing to kill his target.
It's a bit bare, but deliberately so just as the characters are defined rather than explored. That's because the director Richard Fleischer is concentrating attention on what the audience has paid to see: Raquel Welch in a skin-tight wet suit. That, and an incredible array of sets showing bits of the human body blown up to thousands of times their normal size. It isn't often you see the cochlea of an inner ear the size of a cathedral or nerve fibres like bridge cables. Today, this would all have been done with green screen, but this being the '60s, the only option was a mixture of miniatures, process shots, front projection screens, and full-size mock ups for the scenes with actors in them. All of this, of course, occurs inside the human body, so, except for one scene inside the air cavity of the lungs, the characters are swimming about when they leave their submarine. There was no way that the studio was going to compound the cost of the sets by immersing them in a tank, so the "swimming" actors are actually all on wires.
The pacing is a bit slow in the beginning, but that's because of the need to sell what was then a novel concept and make it believable, so every step of the miniaturisation process is followed in detail. However, the Grant character is a brilliant idea because he is a) the fish out of water whom we need to identify with and people get to explain things to, yet b) he's a active agent trying to solve a problem, so he isn't just sitting about asking questions.
Actually, the more brilliant idea was Raquel Welch's character, who wears a skin-tight wet suit.