Fantastic Voyage is a 1966 American science fiction adventure film directed by Richard Fleischer and written by Harry Kleiner, based on a story by Otto Klement and Jerome Bixby. The film is about a submarine crew who is shrunk to microscopic size and venture into the body of injured scientist Dr. Jan Benes to repair damage to his brain. In adapting the story for his script, Kleiner abandoned all but the concept of miniaturization and added a Cold War element. The film starred Stephen Boyd, Raquel Welch, Edmond O'Brien, Donald Pleasence, and Arthur Kennedy.

Bantam Books obtained the rights for a paperback novelization based on the screenplay and approached Isaac Asimov to write it. Because the novelization was released six months before the film, many people mistakenly believed that the film was based on Asimov's book. Its modern and imaginative production design received five nominations at the 39th Academy Awards mostly in technical departments, winning for Best Visual Effects and Best Art Direction in Color.

The film used the concept of miniaturization in science fiction along with The Incredible Shrinking Man and inspired an animated television series of the same name.

The United States and the Soviet Union have both developed technology that can miniaturize matter by shrinking individual atoms, but only for one hour. A scientist, Dr. Jan Benes, working behind the Iron Curtain, has figured out how to make the process work indefinitely. With the help of American intelligence agents, including agent Charles Grant, he escapes to the Western world and arrives in New York City, but an attempted assassination leaves him comatose with a blood clot in his brain that no surgery can remove from the outside.

To save his life, Grant, United States Navy submariner Captain Bill Owens, medical chief and circulatory specialist Dr. Michaels, surgeon Dr. Peter Duval, and his assistant Cora Peterson are placed aboard a Navy ichthyology submarine at the Combined Miniature Deterrent Forces (CMDF) facilities. The submarine, named Proteus, is then miniaturized to "about the size of a microbe", and injected into Benes' body. The team has 60 minutes to get to the clot, remove it, and exit Benes' body; if they do not get out in time, the Proteus and its crew will begin reverting to their normal size, which will either place them under attack from Benes' immune system or kill Benes himself.

The crew faces many obstacles during the mission. An undetected arteriovenous fistula forces them to detour through the heart, where cardiac arrest must be induced to, at best, reduce turbulence that would be strong enough to destroy the Proteus. The crew faces an unexplained loss of oxygen and must replenish their supply in the lungs. They notice "rocks" that are actually carbon particles from smoke. Grant finds the surgical laser needed to destroy the clot was damaged from the turbulence in the heart, as it was not fastened down as it had been before: this and his safety line snapping loose while the crew was refilling their air supply lead Grant to suspect a saboteur is on the mission. The crew must cannibalize their wireless radio to repair the laser, cutting off all communication and guidance from the outside, although because the submarine is nuclear-powered, surgeons and technicians outside Benes's body are still able to track their movements via a radioactive tracer, allowing General Alan Carter and Colonel Donald Reid, the officers in charge of CMDF, to figure out the crew's strategies as they make their way through the body.

The sub enters the lymphatic system, but the reticular fibers start to interfere. The crew is then forced to pass through the inner ear, requiring all outside personnel to make no noise to prevent destructive shocks, but while the crew is removing reticular fibers clogging the submarine's vents and making the engines overheat, a fallen surgical tool causes the crew to be thrown about and Peterson is nearly killed by antibodies, but they are able to reboard the submarine in time. By the time they finally reach the clot, the crew has 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 instead begins to suspect Michaels. During the surgery, Michaels knocks out Owens and takes control of the Proteus while the rest of the crew is outside for the operation. As Duval finishes removing the clot with the laser, Michaels tries to crash the submarine into the same area of Benes' brain to kill him. Grant fires the laser at the ship, causing it to veer away and crash, and Michaels to get trapped in the wreckage with the controls pinning him to the seat, which attracts the attention of white blood cells. While Grant saves Owens from the Proteus, Michaels is killed when a white blood cell consumes the ship. The remaining crew quickly swims to one of Benes' eyes and escapes through a tear duct seconds before returning to normal size.