Thirteen-year-olds prim and proper Bostonian Sharon McKendrick (Hayley Mills) and tomboyish Californian Susan Evers (Hayley Mills) meet at summer camp. It's hate at first sight, as besides the differing length of their hair, they look exactly the same. After the girls carry out one battle after another against each other, the camp administrators ultimately place the two in solitary confinement, the two eating, sleeping, and playing only with each other. It is during this confinement that they finally get a chance to know each other and learn that they are indeed twins, separated when they were babies when their parents, Maggie McKendrick (Maureen O'Hara) and Mitch Evers (Brian Keith), divorced. Wanting to get to know and ultimately love the parent they never met, Sharon and Susan decide to switch places, Sharon will go to California and pretend to be Susan, and Susan will go to Boston and pretend to be Sharon. The other thing they believe is that their parents still love each other, why else had neither ever remarried. The two parents would ultimately have to "un-switch" them, when the girls hope their parents will fall in love all over again. Sharon and Susan are initially able to carry off the ruse, but in California, their plan hits a snag when Mitch introduces his daughter to a young woman named Vicky Robinson (Joanna Barnes), Vicky, who aspires to be the second Mrs. Mitchell Evers. Sharon and the domestics in the Evers household think Vicky is merely a gold digger out for Mitch's money. Sharon and Susan have to figure out a way to get rid of Vicky and bring Maggie to California more quickly than planned, so that their parents can reconcile, and the foursome can be one big happy family.