A chemist and his wife drink his youth formula and act like kids and become jealous while his boss tries to find out what the formula is.
Absent-minded Barnaby Fulton (Cary Grant) tells Edwina Fulton (Ginger Rogers) about his experiments, and they decide not to go to a dance. Hank Entwhistle (Hugh Marlowe) arrives but can’t persuade them to go.
In an office secretary Lois Laurel (Marilyn Monroe) shows Barnaby the stockings he invented. Oliver Oxley (Charles Coburn) discusses an ad with Barnaby. They see a chimp acting wildly after taking Barnaby’s formula. Barnaby says it is a young female, and they all calm down. Barnaby mixes ingredients. Left alone, the chimp mixes liquids, drinks some, and puts it all in the water cooler before being put back in her cage. Barnaby drinks his formula and then drinks water. He feels dizzy and can see without his glasses, feeling young and chipper. Edwina calls, and Barnaby leaves, gets a hair-cut, and buys a jacket and a sports car. Lois finds Barnaby, and he takes her for a wild ride. He hits a truck, and they go skating and swimming. Barnaby drives but can’t see well.
Later Edwina finds Barnaby asleep in his lab. He tells her what he did. She takes a double dose and drinks water. Oxley comes in, and she puts a goldfish down his pants. She gets excited and jealously threatens Lois. Edwina drives fast. They take a bridal suite. Edwina dances with Barnaby. She is energetic while he is tired. She changes in the bathroom and cries like a child. They quarrel jealously. She locks him out, and his belt is caught in the door. He gets lost and falls down the laundry chute.
In the morning Edwina finds Barnaby and drives him home. Reporters ask him about his divorce. She tells lawyer Hank to go home and explains to her mother. Barnaby and Edwina go to his lab, and he asks why she wanted a divorce. She says he strayed with Lois. He destroys his formula, and she uses water to make coffee. Oxley tells Lois to have guards keep them there. Edwina and Barnaby drink coffee and get silly. Oxley tells the directors the formula did not affect him. Barnaby and Edwina come in with the chimp and act like children and play with Lois.
Outside Barnaby and Edwina put paint on each other and go home. Edwina calls Hank and goes to sleep. Barnaby plays Indians with kids. A naked baby wakes Edwina, who thinks he is Barnaby and takes him in a taxi. Kids tie Hank to a tree, and Barnaby dances around him. Edwina tells Oxley that the baby is Barnaby. Oxley calls Barnaby’s assistant Jerome (Henri Letondal). Edwina puts the baby down to sleep. Oxley and directors take a drink with water. Barnaby comes in the window and lays down to sleep. Oxley is affected, and Edwina finds Barnaby. Jerome tells Barnaby it was in the water. Oxley and the directors mess around.
In the final scene Barnaby and Edwina dress up to go out and kiss.
This screwball comedy has fun with the fantasy that a chemical can make one young, but they come to realize that with youth they lack maturity and get into all kinds of trouble acting without inhibitions.