The United States and the Soviet Union have both developed technology that can miniaturize matter by shrinking individual atoms, but only for a limited amount of time, depending on how small the item is miniaturized.
The scientist Dr. Jan Benes(Jean Del Val), working behind the Iron Curtain, has figured out how to make the process work indefinitely. With the help of the CIA, he escapes to the West, but an attempted assassination leaves him comatose with a blood clot in his brain.
To save his life, agent Charles Grant (Stephen Boyd), pilot Captain Bill Owens (William Redfield), Dr. Michaels (Donald Pleasence), surgeon Dr. Peter Duval (Arthur Kennedy), and his assistant Cora Peterson (Raquel Welch) are placed aboard a specially designed submarine at the C.M.D.F. (Combined Miniaturized Deterrent Forces) facilities. The submarine, named the Proteus, is then miniaturized and injected into Benes. The ship is reduced to one micrometer, giving the team one hour (60 minutes) to remove the clot. After the 60 minutes have elapsed, the Proteus and its crew will begin to revert to its normal size, become vulnerable to Benes's immune system, and (in the words of Asimov's novelization) "kill Benes regardless of the success of the surgery."
The crew faces many obstacles during the mission. An arteriovenous fistula forces them to detour through the heart, where cardiac arrest must be induced to avoid turbulence, through the inner ear (all outside personnel have to remain silent to prevent turbulence) and replenish their supply of oxygen in the lungs. When the surgical laser needed to destroy the clot is damaged, it becomes obvious there is a saboteur on the mission. They are forced to cannibalize their wireless telegraph to repair the device. By the time they finally reach the clot, they have only six minutes remaining to operate and then exit the body.
Before the mission, Grant had been briefed that Duval was the prime suspect as a potential surgical assassin. But as the mission progresses, he pieces the evidence together and instead begins to suspect Michaels. During the critical phase of the operation, Dr. Michaels knocks Owens out and takes control of the Proteus while the rest of the crew is outside for the operation. Duval successfully removes the clot with the laser, but Michaels tries to crash the sub into the clot area to kill Benes. Grant fires the laser at the ship, causing it to veer away and crash. Michaels is trapped in the wreckage and killed when white blood cells attack and destroy the Proteus. Grant saves Owens from the ship and they all swim desperately to one of Benes's eyes, where they escape through a tear duct seconds before returning to normal size.
In the original screenplay, there was a follow-up scene where we learn that, because of brain damage caused by the submarine, Benes no longer remembers the formula for unlimited miniaturization. Surviving stills suggest that this scene was filmed but never used, and the film does not tell how the Proteus failed to return to normal size.
What formula does Benes no longer remember after the brain damage?
Formula for unlimited miniaturization