I read my copy at least 3 decades ago, so for a reasonable plot summary, I rely on a review by my favorite reviewer (from Analog September 1966):
"To get the credits straight, this is a novel by Isaac Asimov based on a screenplay by Harry Kleiner based on a story by Otto Klement and Jay Lewis Bixby (who used to be known - and favorably - in these pages as "Jerome" Bixby). The film may have been released by the time you read this: I admit I'm very curious about how much latitude the good Dr. Asimov really had. Maybe he'll tell us, off the record, at the World Science Fiction Convention in Cleveland, Labor Day weekend.

On the plus side, this is a fast-moving adventure yarn of early Gernsback (a course in every chapter) vintage. On the minus side, it attempts to be a mystery that just isn't mysterious. It also resurrects that old Ray Cummings gimmick, Type B, the descent into smallness.

A physicist named Benes has defected from an unnamed Iron Curtain country (Twentieth Century Fox doesn't want to jeopardize European distribution of the film by being too explicit), bringing a fabulous secret with him. "They" try to kill him and succeed only in damaging him: a blood clot in his brain will destroy his memory if it doesn't kill him. So a mismated crew is miniaturized and sent through Benes' bloodstream in a submarine to find and blast away the clot with a laser. Two doctors, one beautiful technologist, one physicist, one CIA agent... one of them a saboteur. And they have just one hour, normal time, before the miniaturization wears off. If they're not done and out by then, sub and crew will return to normal size inside Benes' brain...

The real perils are in the fantastic voyage through the blood stream, lymph ducts, lungs, inner ear and other portions of the body, shaken by the turbulent flow of the blood in which they are drifting, threatened by voracious white cells, attacked by antibodies, with the clock ticking back toward zero. The minor hazards are the saboteur's rather clumsy efforts to louse things up so they will turn back. Since he is forever insisting they do just that his identity is no surprise - unless it's supposed to be a switch to the most probable suspect.

From the scenes from the film in the Saturday Evening Post condensation, it should be a real thriller! I wish the Good Doctor had been in on it from the beginning." [-P. Schuyler Miller.]