A specialist crew including CIA agent Grant (Boyd), pilot Bill Owens (William Redfield), circulatory specialist Dr. Michaels (Donald Pleasence), surgeon Dr.

Peter Duval (Arthur Kennedy) and his assistant Cora Peterson (Welch) board a submarine together. They are then miniaturised and injected into the bloodstream of scientist Dr. Jan Benes (Jean Del Val), who was shot while defecting from the Soviet Union to the United States, in order to carry out delicate, life-saving surgery.

Travelling though his body is hazardous, but far more dangerous is the suspicion that one of them is a traitor...