Fitzhugh and Barry run across a giant boy, Jodar, being chased by a giant hillbilly. They help him escape. Jodar is a boy genius, kind of a prototypical Wesley Crusher, and gives them an enlarging pill he's been working on. Chipper gets hold of it and it works!
Fitzhugh returns later and takes the pill, grows to giant size, and steals some clothes from the hillbilly. The rest of the Earthlings want nothing to do with him, and since there is no antidote he has no way to go back with them once they fix the ship. Disgruntled, Fitzhugh goes off and intimidates some giants. That, the Earth money in his pocket, and the clothing which match the description of the hillbilly (who has been attacking children) get him arrested.
Fitzhugh fakes amnesia, but Inspector Kobick isn't buying it. Steve contacts Jodar and enlarges himself, then pretends to be a respected lawyer. Kobick sees through the ruse and imprisons him as well. Jodar comes up with an antidote, and both Steve and Fitzhugh shrink and escape. To make sure Fitzhugh isn't arrested again if he grows back to giant size (??), they go to the hillbilly's house and find evidence that convinces the police to arrest him for the assault on the kids. They also give the remainder of the antidote to Chipper, while Jodar departs for college.