Dr. Barnaby Fulton, Cary Grant, is a slightly absent-minded research chemist at the Oxly chemical company who dreams of an elixir of youth that could renew aging bodies. His boss, the financially shrewd Oliver Oxly, pushes him toward a breakthrough that could be a blockbuster, and the board shares the same keen interest in turning science into profit. The laboratory atmosphere buzzes with ambition, but chaos soon arrives in the form of Esther, a clever chimpanzee, who breaks loose and, in a moment of mischief, sprinkles a beaker’s contents into the water cooler. The mixture is not just a random accident — it holds the rejuvenating power Barnaby has been chasing.
Unaware of Esther’s antics, Barnaby tests the latest experimental concoction on himself, taking a sip from the infected water and paying the price with a startling transformation: he instantly regresses to the vitality of a twenty-year-old. With new vigor, he spends the day in the town’s bustling scenes in the company of Lois Laurel, [Marilyn Monroe], who brings a bright, breezy energy to the adventure as she becomes his companion on this improbable journey. The world looks new through Barnaby’s eyes, and the pair find themselves drawn into a day of light, risky fun that feels like a rebirth rather than a reproof of age.
Back home, Edwina Fulton, [Ginger Rogers], Barnaby’s wife, discovers the serum’s remarkable effects and drinks the potion along with water from the cooler, turning into a prankish, impulsive schoolgirl with a taste for mischief and a spark of rebellion. Her transformation jolts the household, and she reaches out to Hank Entwhistle, her old flame and family lawyer, in a quick, impulsive phone call that adds another layer of comedy and tension to the plot. Meanwhile, Edwina’s mother remains clueless about the chemical influence, concluding only that her daughter is unhappy in her marriage and might be seeking a separation.
The next morning, Barnaby and Edwina rush to the laboratory to destroy the dangerous formula, hoping to end the mayhem. Ironically, they become unwitting victims of their own experiment once again, as they drink from the contaminated water cooler to make coffee. Now under the influence, both Barnaby and Edwina slip further into their respective playful, childlike personas, while Oxly and the board step up their efforts to acquire the formula while Barnaby remains under its spell.
A scene of broader mayhem unfolds as Barnaby cavorts with a group of children who pretend to be Indians, adding a touch of whimsy and innocence to the chaos. At the same time, Edwina experiences a startling sleep and awakens to find a baby beside her. Convinced that Barnaby has overdosed, she panics, only to learn that the infant is the responsibility of the Fulton housekeeper. The duo frantically search for the antidote, and Edwina—ever determined—tries to coax the baby to sleep, hoping this will reverse the effects of the elixir.
As the laboratory becomes a hub of experimentation, more scientists and Oxly himself drink from the water and regress into a second childhood, creating an escalating cycle of confusion, humor, and gentle chaos. Barnaby, slipping through the lab window, crawls inside and settles beside the baby, exhausted but at peace in the shared stillness. Edwina discovers him, recognizing the mistake but also the fragile bond that remains between them.
In the culmination, the household homecoming reframes their relationships. Barnaby, now back in control after a moment of uncertain vulnerability, has been offered a new contract by Oxly, signaling a practical prize for his scientific talent. As he prepares for an evening with Edwina, the couple’s spirits lift and their bond seems renewed. The film closes on a sly, hopeful note about youth and aging, encapsulated in Barnaby’s final reflection: > you’re old only when you forget you’re young.
