Wealthy American Jervis Pendleton III (Fred Astaire) has a chance encounter at a French orphanage with a cheerful 18-year-old resident, Julie Andre (Leslie Caron). He anonymously pays for her education at a New England college. She writes letters to her mysterious benefactor regularly, but he never writes back. Her nickname for him, “Daddy Long Legs”, is taken from the description of him given to Andre by some of her fellow orphans who see his shadow as he leaves their building. Several years later, he visits her at school, still concealing his identity. Despite their large age difference, they soon fall in love.

Barnaby Fulton is a research chemist working on a fountain of youth pill for a chemical company. One of the labs chimps, Esther, gets loose in the laboratory and, having watched Dr Fulton do something similar, mixes a beaker of chemicals, but then pours the mix into the water cooler. Later, when trying one of his own samples sample, washed down with water from the cooler, Fulton begins to act just like a 20-year-old and believes his potion is working. Soon his wife and boss are also behaving like children.