When song-writer Alex Burke (Gig Young) enters the lives of the musical Tuttle family, each of the three daughters falls for him. The family lives in the fictional town of Strafford, Connecticut. Alex's personality is a match for Laurie Tuttle (Doris Day), as both she and Alex are seemingly made for each other. Her two sisters, although seeing other men, are infatuated with him as well. Soon Laurie and Alex are engaged, but when a friend of Alex's, Barney Sloan (Frank Sinatra), comes to the Tuttle home to help with some musical arrangements, complications arise.
His bleak outlook on life couldn't be any more contradictory to that of Alex's, but Laurie is infatuated with him. That infatuation leads her to run off with Barney on what was supposed to be her wedding day to Alex.
Meanwhile, Laurie's two other sisters, Fran (Dorothy Malone), and Amy (Elizabeth Fraser) each marry, despite still having feelings for Alex.
Barney, with a black cloud perpetually hanging over his head, decides one evening to kill himself, feeling Laurie would be better off with Alex, as he would be a better provider. Barney drives into oncoming traffic during a snowstorm with his windshield wipers off. But he lives, and with a new found affirmation of life, and he finally writes the song he had been working on, finding his self-esteem in the arms of Laurie.
The character of the self-destructive Barney Sloan was originally written to die at the end of the film when Sloan drives into on-coming traffic during a snow-storm. Sinatra, whose characters in his two previous films From Here to Eternity (1953) and Suddenly (1954), filmed before but released after Young at Heart perished at the end, thought Sloan should live and find happiness. Sinatra's growing influence in Hollywood was enough to have the ending re-written to accommodate his wishes thus creating a corny instead of a believable ending. Happy endings triumph once again.