The story unfolds as a flashback, told by The Ancient, [Harry Davenport], a fairy who sits at the base of an ancient tree that houses a close-knit group of magical beings. The youngest creatures doubt that humans truly exist, so The Ancient shares a tale about the power of belief and how it can shape fate.
In 1870, Rena Fairchild, [Cyd Charisse], a young American woman, is courted by three men: Richard Gaunght [Lionel Barrymore], James Trumbell [Lewis Stone], and Theodore Findley [Edward Arnold]. She is swept off her feet by a dashing Irishman, the O'Monahan, [Warner Anderson], and travels with him to Ireland, but not before the O'Monahan pronounces a blessing on the three, foretelling that each will achieve his materialistic dreams.
Forty years pass, and the trio live in a grand house, each having found success in his field—medicine for Gaunght, the law for Trumbell, and banking for Findley—yet they are not particularly beloved by their community. They donate some land to the local university, not purely out of generosity, but to polish their public image and endear themselves to others.
Enter Sheila O'Monahan, [Margaret O'Brien], the orphaned granddaughter of Rena, arriving with Terence Aloysius O'Davern, [Thomas Mitchell], the longtime family servant, and informing the three men that they are to accept her as an adopted goddaughter. At first they resist, but they discover an embarrassing truth: Sheila is the rightful heir to the land they donated, which actually belongs to her. They take Sheila and O'Davern into their household, without explaining the full reason behind their sudden change of heart.
Sheila believes in Irish magic and the supernatural beings said to inhabit the old tree, invoking them in Gaelic. Her faith begins to win over Gaunght and Trumbell, while Findley schemes to reclaim the land and its power. When they reveal plans to build an amphitheater on the site—an effort that would require felling the ancient tree—Sheila rejects the idea. Findley retaliates by hiring a troupe of performing dwarfs to pose as pixies, pretending the tree is dying and that the pixies are leaving.
When Sheila uncovers the hoax with O'Davern’s help, her faith in people, as well as in pixies, falters. She runs away to a convent, announcing her intent to become a nun to the Mother Superior. Meanwhile, Gaunght chains himself to the old tree to prevent its destruction, and Trumbell joins him. After the Mother Superior confronts them, the three men, now remorseful, accompany Sheila near the tree and declare that they can now see the “little people” themselves—though only Sheila truly sees them. In a warm, renewed sense of family, they all decide to adopt her, and the tree is spared.
