Cocky navy test pilot Tuck Pendleton (Dennis Quaid) undergoes a top-secret miniaturization experiment and is accidentally injected into the body of a hypochondriac clerk, Jack Putter (Martin Short). With Tuck floating inside his body, Jack attempts to outwit saboteur Victor Scrimshaw (Kevin McCarthy), who intends to sell the miniaturization technology to a nameless criminal known as "The Cowboy" (Robert Picardo).