One afternoon, famed author and genius Lynn Belvedere is walking in the park when he overhears four elderly people complain about their bleak lives. Belvedere, whose current lecture tour is on the topic "How to Be Young Through Eighty," is intrigued by the idea of entering the Church of John Home for the Aged, where the four oldsters live, and helping them to regain their vigor and enjoyment of life. Telling his manager, Emmett Wilson, that he will join him later, Belvedere goes to see Bishop Daniels, the nominal head of the home. The bishop assumes that Belvedere is Oliver Erwenter, who recently applied for admission to the home, and Belvedere does not correct him. Belvedere's youthful appearance bewilders the bishop, as Erwenter's birth certificate states that he is seventy-seven years old, but Belvedere talks his way into being accepted. At the home, Belvedere meets the four people on whom he had eavesdropped earlier: Mr. Beebe, Mr. R. B. Cherry, Mrs. Hammer and Mrs. Gross, as well as nurse Harriet Tripp and several other residents of the rundown facility. Reverend Charles Watson, who is the home's administrator, is upset to learn that they have a new resident, as the trustees have just informed him that they will soon be halving the already meager budget. Belvedere's officious, egocentric attitude annoys Charles, although Harriet, who is in love with Charles, is herself irritated by Charles' refusal to stand up to the trustees and demand more money. As Belvedere settles into his new quarters, his lively manner intrigues the residents, and he thrills Beebe and Cherry when he tells them that his youthful appearance is due to a secret Tibetian potion. Later that evening, Belvedere woos Harriet in an effort to make Charles jealous, and when Harriet sees Charles watching from his study, she plays along. The next morning, an infuriated Emmett finds Belvedere and demands to know why he did not appear at his last lecture. Belvedere explains that he wants to make the residents young again, and drags Emmett along as he goes to town. There, Belvedere prepares some placeboes at the pharmacy and has them mailed to himself at the home. Upon Belvedere's return, Charles chastises him both for his romance with Harriet and his "upsetting" influence on the other residents. Belvedere insists that his fresh attitude is good for his new acquaintances, then accompanies them on a picnic. Harriet is pleased by the liveliness of the formerly sedate residents and agrees to Belvedere's suggestion that they hold a church bazaar to raise funds. Charles, who has discovered Belvedere's true identity, does not reveal his knowledge upon seeing how happy Belvedere has made everyone. Later that night, Belvedere instructs Emmett to steal furniture and decorations for the garden, and the residents are amazed by the improvement in their surroundings when they awaken in the morning. Beebe and Cherry, who have taken some of Belvedere's placebo pills, are convinced that they are indeed growing younger. Even Charles begins to believe in Belvedere, and thanks him for his help. Belvedere's plans unravel, however, when a group of reporters discover him and reveal his identity to his friends. The new garden furnishings are reclaimed by their irate owners, and Beebe, Cherry and the others turn against Belvedere and become depressed. Harriet tries to convince them that Belvedere was trying to help, but they do not listen. Determined not to give up, Belvedere enlists Charles and spends the night buying back the furnishings and arranging for the bazaar. The next day, the delighted residents enjoy their new supplies and welcome visitors to the bazaar. Charles, who had been praised by Harriet for not revealing Belvedere's identity earlier, is inspired by Belvedere to propose to Harriet, and she happily accepts. With his work accomplished, Belvedere leaves with Emmett but frostily denies that he is "lightening up."