Summary:

The Cold War. America and Russia (or rather "us" and "them") have both developed miniaturization technology that enables them to reduce objects, including human beings, to microscopic size. The Americans are unable to control the objects’ return to normal size after an hour; the Russians can. An American spy called Benes has stolen this information from the Russians but on his return to America he is injured when the Russians try to kill him. He develops a blood clot in his brain.

To remove the clot, a team of western scientists, led by the surgeon Duval (Edmond O’Brien) and a British vascular specialist, Michaels (Donald Pleasance), is miniaturized inside a submarine which is injected into Benes’s carotid artery. Dr. Duval has a laser gun with which he is to destroy the clot. Also on the submarine are Grant (Stephen Boyd), a military employee in charge of security, and Cora Petersen (Raquel Welsh), Duval’s technical assistant.

The team has an hour to reach the patient’s brain and destroy the clot. They overcome various hurdles, including being washed through an arterial-venous fistula in the jugular vein, having to travel through Benes’s heart (which is temporarily arrested by the outside surgical team to keep the submarine from being crushed), being attacked by antibodies in the lymphatic system, and having to replenish their air supply by breaking through the wall of an alveolar sac.

Finally, they reach the brain and find the clot, but Dr. Michaels turns out to be spying for the other side, and tries to sabotage the mission. He crashes the submarine, but is thwarted by Grant and ingested by a white blood cell. Duval destroys the clot and the crew escapes Benes’s body via the optic nerve. They are washed out in a tear just as they are beginning to return to normal size. Benes is never seen to wake up, but the film’s ending implies that the mission has been successful.

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