Hayley Mills demonstrates considerable talent in this energetic film directed by David Swift. She portrays the upper-class "Sharon" and her less refined sister "Susan," who find themselves at one of those summer camps where parents send children they don’t want during school breaks. As expected, the sisters dislike each other intensely, and after creating problems for nearly everyone around them, they are put in time-out together. It's while they’re separated that their relationship begins to improve, leading them to discover they are twins who were split up following their parents’ divorce. They devise a scheme – sending the incorrect sister home with each parent. "Sharon" is sent to her father, "Mitch" (Brian Keith), and “Susan” goes to her proper and reserved mother, "Maggie" (Maureen O'Hara). Their goal is to reunite their parents and create happy families. "Sharon" encounters a more difficult situation when she realizes that Mitch’s friend, "Vicky" (Joanna Barnes), wants to marry him, so the girls must bring their mother from Boston to California for their plan to work. The storyline is quite unbelievable, but Mills handles her dual roles effectively; the initial conversations are amusing and the antics of the sisters do elicit some laughter. Unfortunately, the performances of the adult actors weigh down the movie—particularly O’Hara, who appeared uncomfortable in her part. Keith wasn't known for his versatility, and he does an adequate job here, but as the grown-up drama takes over the narrative, I began to feel tired because it seemed to move slowly toward its two-hour length.

Source on TMDB