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Chemist Dr. Barnaby Fulton (Cary Grant) is developing a pill that will defy the aging process for the pharmaceutical company run by Oliver Oxley (Charles Coburn). When a loose chimpanzee mixes chemicals together that produce this effect, Fulton tries some on himself. This prompts him to act like a teenager, making passes at Oxley's buxom secretary, Lois (Marilyn Monroe). Soon everyone, including Fulton's wife, Edwina (Ginger Rogers), is feeling the effects of the formula, with hilarious results.