A maverick pilot, Tuck Pendleton, volunteers for a secret miniaturization experiment where scientists successfully shrink him and his submersible craft to microscopic size. The intention is for him to be injected into a lab rabbit, but a rival organization launches a surprise attack intent on stealing this revolutionary technology. In the ensuing chaos, a lab technician flees with the syringe containing the miniaturized Pendleton and, in a desperate attempt to avoid his pursuers, injects him into an unsuspecting grocery store clerk, Jack Putter.

Thrown together by extraordinary circumstances, Tuck must now navigate the bewildering landscape of Jack's body, communicating with the timid clerk through a device implanted in his ear. As Tuck guides Jack to evade their pursuers, the two forge an unlikely alliance to retrieve the stolen microchip that can re-enlarge Tuck before his oxygen runs out. Unbeknownst to them, Tuck's journalist girlfriend has also become embroiled in the dangerous espionage surrounding the groundbreaking technology. The trio must race against time while facing off against the sinister forces aiming to claim the micro-miniaturization process for their own nefarious purposes.