The top-secret test of a sub-miniaturization process goes wildly awry in this comic adventure when industrial spies steal the technology and the daring pilot (Dennis Quaid) of the world's first molecular-sized craft is accidentally injected into the bloodstream of a hypochondriac grocery store clerk (Martin Short). Now with only a 24-hour supply of oxygen, an atom-sized test pilot must convince the nerd in whose body he is trapped how to be a hero in order to recover the stolen technology, convince the pilot's girlfriend (Meg Ryan) that he loves her ... and save both men from a grisly death.