A scientist's search for the fountain of youth makes him and his wife regress to childhood.

Absentminded scientist Barnaby Fulton is discouraged when his formula, which he hopes will reverse the ill effects of aging, proves ineffective on his test chimpanzees, Rudolph and Esther. Barnaby is encouraged by his wife Edwina, who loves her brilliant husband despite his eccentric behavior. One morning, when Barnaby arrives at the Oxly Chemical Factory, where he works as a free-lance scientist, he is summoned to president Oliver Oxly's office. The elderly Oxly hopes that Barnaby's formula will be financially successful, and will enable him to pursue his curvaceous secretary, Lois Laurel. Oxly shows Barnaby the ad campaign he has devised for the formula, which he has named B-4, but Barnaby urges him to be patient. Oxly's hopes are raised when it appears that the aged Rudolph is bounding around like a youngster, but it is revealed that Rudolph's numbered uniform was inadvertently switched with that of the younger, mischevious Esther. Barnaby begins work, but when he leaves the laboratory, Esther frees herself, and copying Barnaby, mixes a batch of the ingredients for his formula. Just before Barnaby returns, Esther pours her chemicals into the water cooler, after which Barnaby mixes his own batch. Against the wishes of his assistant, Dr. Jerome Lenton, Barnaby swallows the formula, and its bitter taste forces him to get some water from the cooler. Soon Barnaby notices that his bursitis no longer hurts, and that he can see without his thick glasses. Overcome by a feeling of euphoria, Barnaby leaves the laboratory and has his mature haircut changed to a more youthful crewcut, then buys a loud, checked sportscoat. Next, at the car dealership, Barnaby purchases a sports car, and asks Miss Laurel, who has been sent to find him, to go for a ride. Barnaby flirts with Miss Laurel as they race, then roller skate and swim, and she rewards him with a kiss on the cheek. She pouts upon discovering that Barnaby is married, however, and after his return to the lab, Barnaby falls asleep. When he awakens, the effects of the formula have worn off, although the just-arrived Edwina is nonplussed by the tale of his antics, as well as the lipstick on his cheek. Barnaby, who states that the formula made him act like a twenty year old, assures Edwina that the kiss meant nothing, but in order to keep her husband from experimenting on himself again, Edwina takes the next batch of formula he concocts. Edwina drinks a cup of water and soon she, too, is acting like a twenty year old. Edwina and Barnaby elude the inquiring Oxly, after which Edwina demands that they go to the hotel where they honeymooned. At the hotel, Edwina's energy exhausts Barnaby, and when they finally prepare for bed, Edwina, acting like a new bride, begins to cry for her mother. The couple are soon quarreling, and when Barnaby gently pushes Edwina away from his fallen glasses, she throws him out of their room and calls their lawyer, Hank Entwhistle, to tell him that Barnaby has brutalized her and that she wants a divorce. Blind without his glasses, Barnaby winds up in the laundry room, where he spends the night. In the morning, the fully recovered Edwina takes Barnaby home and tells Hank, who is in love with her, that she wishes to call off the divorce. Disheartened by Edwina's brief wish to leave him, and her revelation that Hank had once kissed her, Barnaby decides that his formula causes only chaos and should be destroyed. At the lab, Edwina uses the cooler water to brew a pot of coffee, and after several cups, she and Barnaby begin to act like ten year olds. Oxly, who has heard about the success of the formula, calls in the board of directors and urges them to offer Barnaby anything he wants for the rights. Barnaby and Edwina are then brought to the meeting, where Barnaby asks for a "zillion" dollars for his formula. Barnaby and Edwina escape the confines of the lab, but as they walk home, Edwina's attempts to play annoy the now girl-hating Barnaby. Barnaby then runs off to join some youngsters who are playing "Indian," while Edwina returns home and calls Hank to complain about Barnaby, then takes a nap. As Edwina sleeps, Barnaby dresses like an Indian and talks his little cohorts into scalping Hank, of whom he is still jealous. When a recovered Edwina awakens, she discovers that a neighbor's baby, Johnny, has crawled into her bed, and mistakenly assumes that Barnaby has taken an overdose of the formula and reverted to infancy. While Edwina dashes to the laboratory with Johnny in her arms, Barnaby and his pals succeed in capturing Hank and cutting his hair into a mohawk style. At the laboratory, Oxly, Lenton and the other astonished scientists gather around Johnny, whom they, too, believe to be Barnaby. Edwina lays the baby down on Barnaby's office couch, hoping that if he sleeps, he will return to normal. While the men pace outside, they drink water from the cooler, and Oxly orders that the bitter-tasting water be thrown out. Barnaby then climbs through the window to his office, and Edwina soon finds him. Realizing her mistake, Edwina laughingly greets her recovered husband, then goes with him to the outer laboratory. There, they are amazed to see Oxly, the board members and the scientists acting like children, until Lenton deduces what Esther had done. Content to leave Oxly chasing Miss Laurel with a seltzer bottle, the Fultons leave. Three days later, with a new Oxly contract ensuring his future, a romantic-minded Barnaby tells Edwina that a person is old only when he forgets that he is young.