Jack Putter was an extremely nervous hypochondriac who worked as a cashier at Safeway. He had an on-again, off-again relationship with fellow employee Wendy. Told by his physician Dr. Greenbrush that he needed some vacation time, he went to a travel agency to get tickets for a cruise. There he encountered a dying scientist who injected something into his posterior as his final act. That something turned out to be Navy pilot Tuck Pendleton in a miniaturized submarine.
Tuck soon made contact with Jack and explained his situation, causing Jack to, initially, think he was possessed (Dr. Greenbrush soon reassured him that this was not the case). At first, Jack wanted nothing to do with the tiny man inside his body, preferring to ignore him. However Tuck's pleas that he would soon run out of air unless returned to normal size prompted Jack to finally step up and do the right thing, to say nothing of constant attacks by the henchmen of the wealthy Victor Scrimshaw.
Acting on Tuck's instructions, Jack met with Lydia Maxwell in the hopes of discovering what it was Scrimshaw wanted. Following a failed kidnapping at the hands (or hand) of Mr. Igoe, Jack learned that Scrimshaw sought the computer chips required for miniaturization. One of which was in the submarine currently inhabiting Jack's bloodstream. Scrimshaw already possessed the other chips, and because both were required to re-enlargement, Jack was forced to disguise himself as the Cowboy (with Tuck stimulating his nerves to temporarily alter his face).
At first the plan went well, until Jack's nervousness in the presence of Mr. Igoe's blowtorch-hand caused the nerve stimulation to wear off, revealing "the Cowboy's" true identity. Scrimshaw had Jack brought to Dr. Canker's laboratory where a shrunken Igoe was injected into Jack so as to retrieve the other chip. Lydia appeared and, using a gun stolen from Scrimshaw's sunglasses-wearing henchman, rescued Jack. With the help of Pete Blanchard, they were able to get Jack to the lab of Dr. Niles. En route, Tuck fought and killed Igoe inside of Jack's stomach, ending the threat.
Once at Niles' lab, Jack sneezed the shrunken sub out onto Niles' glasses, and then the miniaturized vehicle was placed inside the shrinking machine. The miniaturization process was reversed, seconds before Tuck would've run out of air. Later, Jack quit his job and ended his relationship with Wendy, and attended Tuck and Lydia's wedding where he gave them the cruise tickets he'd bought earlier. As they left in a limousine, he noticed that the chauffeur was actually the Cowboy (the real Cowboy) in disguise, and raced off to rescue his friends in Tuck's Mustang convertible.