Scientist Jan Benes, who knows the secret to keeping soldiers shrunken for an indefinite period, escapes from behind the Iron Curtain with the help of CIA agent Grant. While being transferred, their motorcade is attacked. Benes strikes his head, causing a blood clot to form in his brain. Grant is ordered to accompany a group of scientists as they are miniaturized. The crew has one hour to get in Benes's brain, remove the clot and get out.

Isaac Asimov was approached to write the novel from the script. He perused the script, and declared the script to be full of plot holes. Receiving permission to write the book the way he wanted, delays in filming and the speed at which he wrote saw the book appear before the film.

[as the submarine enters the brain]


Dr. Duval:
Yet all the suns that light the corridors of the universe shine dim before the blazing of a single thought...


Grant:
- proclaiming in incandescent glory the myriad mind of Man...


Dr. Michaels:
Very poetic, gentlemen. Let me know when we pass the soul.


Dr. Duval:
The soul? The finite mind cannot comprehend infinity - and the soul, which comes from God, is infinite.


Dr. Michaels:
Yes, well, our time isn't.

Crew or equipment visible: Shadow visible on the right hand wall as the camera pulls back to see the large control room for the first time[widescreen only].