Cary Grant plays Dr. Barnaby Fulton, an absent-minded professor type who is trying to develop an elixir of youth, urged on by his commercially minded boss (Coburn). One of his chimpanzees gets loose in the laboratory and pours some chemicals into the water cooler — chemicals that just happen to have the rejuvenating effect for which Fulton is searching. Unaware of the monkey's antics, Fulton tests his latest experimental concoction on himself, and washes it down with water from the cooler. Naturally, he soon begins to act just like a 20-year-old, and spends the day out on the town with his boss's secretary (Monroe). When Fulton's wife Edwina (Rogers) learns that the elixir "works," she drinks some, again washing it down with water, and turns into a prank-pulling schoolgirl. Things get out of hand when her newly quick temper induces Edwina to make an impetuous phone call to her old flame Hank Entwhistle (Marlowe), who, knowing nothing of the elixir, believes that Edwina is truly unhappy in her marriage and wants a divorce. Meanwhile, more and more people at the laboratory are drinking the water and reverting to a second childhood, with predictably hilarious results. In the end, of course, everything works out, with help from the elixir itself.
The film is reminiscent of Bringing Up Baby (1938), which also starred Cary Grant and was directed by Howard Hawks, but had a leopard instead of a chimpanzee. The denouement, involving a chemical that causes a board of directors to act like schoolchildren, is shared by 1961's Lover Come Back, a Doris Day–Rock Hudson vehicle, although in that film the chemical — in pill form — simply causes everybody to get extremely drunk.
Hawks said he didn't think the film's premise was believable, and as a result thought the film was not as funny as it could have been. Peter Bogdanovich has noted that the scenes with Cary Grant and Marilyn Monroe work especially well and laments that Monroe wasn't the leading lady instead of Ginger Rogers.