Identical twins Susan Evers and Sharon McKendrick (Hayley Mills) meet at summer camp, unaware that they are sisters. Their identical appearance initially creates rivalry, and they continually pull pranks on one another. Eventually, their mischief ruins the camp dance. As punishment, they must live together in an isolated cabin for what remains of the summer. After both admit they come from broken homes, they soon realize they are twin sisters and that their parents, Mitch and Maggie (Brian Keith and Maureen O'Hara), divorced shortly after their birth, with each parent having custody of one of them. The twins, each eager to meet the parent they never knew, switch places. While Susan is in Boston masquerading as Sharon, Sharon goes to California pretending to be Susan.