Chris Hammond is a high school senior. He likes a girl at school, Lori Beaumont, Camille Cooper who is dating his rival and bully Rick Micah Grant. His father, Dr. Jack Hammond, Dudley Moore is a surgeon who is trying to get promoted to chief of staff at his hospital. Chris has a friend, Clarence “Trigger” Sean Astin, whose Uncle Earl had his leg fixed by Native Americans with a body-switching potion called the “Brain-Transference Serum.” Trigger shows Chris how the serum works by trying it out on Chris’ cat and dog, and the pets switch bodies. Trigger brought the serum in a Tabasco sauce bottle, and the Hammonds’ housekeeper Phyllis, Maxine Stuart puts the bottle in the food cupboard. Jack unwittingly puts some serum in his Bloody Mary. The serum works by someone ingesting it, then the next person that looks into their eyes switches bodies. As Jack looks into his son’s eyes, the father and son switch bodies.
Trigger states that he will get in contact with his Uncle Earl to find a way for the two to switch back, but Earl has just left for another trip. Chris goes to town in his dad’s body, using his dad’s credit card to shop and party with Trigger. He bumps into his dad’s boss’s wife while out in a bar but does not realize who she is. In the morning, Jack in his son’s body and Chris in his father’s body wake up screaming and realize they are not dreaming. Jack goes to school in Chris’s place, while Chris calls in sick at the hospital. Jack’s knowledge of the schoolwork and his willingness to point out troublemakers in class has him shunned by his fellow students. He takes Lori to a concert but does not enjoy himself, finding the music too loud. Then he fails to perform at the big relay race, and Rick beats him up because of the track meet and taking Lori out.
They finally get in touch with Earl, who explains that they can get the antidote if they go on a trip to Death Valley. After a few hiccups, they find the key ingredient for the antidote. Earl makes it up and they drink it; however looking into each other’s eyes does not work immediately. Earl explains it can sometimes take a while to work. The antidote finally works as Jack in Chris’s body is running late for a meeting. Chris in Jack’s body is on his way to a meeting about his dad becoming Chief of Staff, which will not happen now that Jack’s boss found out what Chris did with his wife while in Jack’s body. Now back in their own bodies, both of them race to the hospital.
Chris speaks up at the Chief of Staff meeting to try to persuade the boss to give his dad the job, but his boss will not hear it. Jack walks in and says he does not want the job anymore; he would rather spend the overtime with his son. On the way out, Trigger sees Rick and gives him the Brain-Transference Serum. The next person who looks into Rick’s eyes is Dr. Larry Armbruster. The film ends as they both scream after they switch bodies.
