Minnie visits Mickey while he is playing a video game and is upset to find that he has forgotten the anniversary of their first date due to his addiction to the game. Mickey comes up with the last-minute idea to take her to a miniature golf course for their anniversary and shows her a newspaper ad for it, but she instead notices an ad for a trip to Hawaii, which would cost $999.99, and mistakes it for Mickey's gift. Mickey frets over how he can make enough money for the trip when Pluto (voiced by Bill Farmer) shows him an ad to work with a mad scientist named Dr. Frankenollie (voiced by Kelsey Grammer) for a day of "mindless work" that would pay $999.99. Upon reaching the home of the simian Dr. Frankenollie, Mickey is dropped down a trapdoor into Frankenollie's laboratory, where the doctor plans to switch Mickey's brain with that of the 15 foot tall Julius (voiced by Jim Cummings and modeled after Pete). The experiment causes an explosion that kills Frankenollie, but the brain transfer is a success, with Mickey's mind ending up in Julius' giant body and Julius in control of Mickey's body that makes him have a werewolf-like behavior.
The dimwitted and insane Julius finds Mickey's wallet and notices a photo of Minnie, whom he instantly becomes smitten with. He escapes from the laboratory and finds Minnie while she is shopping for swimsuits and mistakes Julius for Mickey. Mickey arrives in Julius' body to save Minnie, but Minnie mistakes Mickey for a monster (due to his looks) and screams for help until Mickey convinces her of who he is. Julius continues to pursue Minnie, leading to a battle between Mickey and Julius during which they land on a telephone line and are electrocuted, which causes their minds to switch back to their original bodies. Mickey manages to tie Julius up with a rope atop a 100 story building and rescue Minnie. Mickey finds a billboard to the Hawaiian vacation getaway to make Julius be a yo-yo. The two travel to Hawaii together on an inflatable boat pulled by Julius as he swims after the photo of Minnie in Mickey's wallet, which is attached to a fishing line manned by Mickey.