A medical crew (Stephen Boyd, Raquel Welch, William Redfield, Arthur Kennedy, and Donald Pleasence) and a submarine are miniaturized to remove a blood clot from the brain of a Czech scientist who was shot while defecting. Once shrunk and inside the body, the crew battle white corpuscles
while the heart and lungs also make their journey rougher. Early on, it becomes apparent that one of the crew members is a double agent, and matters become even more complex as they race against the clock to complete their mission before returning to normal size.
Their voyage through the body's bloodstream past assorted organs was created by inventive special effects that make t...