The Fantastic Voyage (1985)

Only one scientist in the world knows the secret of miniaturising a living human and prolonging that state for more than 60 minutes. So important is this knowledge that he is rushed by Secret Service men to the top-secret Pentagon-like subterranean H.Q of The Combined Miniature Deterrent Forces. En route, enemy saboteurs stage a car crash in which the Scientist suffers a critical brain injury which can only be successfully treated by operating from inside his head. There follows a suspense-filled race against time. You are selected to undergo miniaturisation and enter the Scientist's body, but you only have 60 minutes. You are placed inside his body in a specially designed submarine which is injected into the Scientist's bloodstream, but the process is too rushed for the submarine to withstand the miniaturisation! Eventually you find yourself in the Scientist's mouth, with parts of the submarine scattered throughout his anatomy. Now your only hope is to discover the eight pieces of submarine scattered around the body and assemble them in the brain. Only then will you ever emerge once more into the outside world. You must also ensure that the Scientist does not die whilst you are inside his body or you will also die.