The Parent Trap
Sharon McKendrick and Susan Evers are identical twins whose parents were divorced when the girls were infants. Sharon has lived with her mother, Maggie, while Susan has lived with her father, Mitch; neither knows of the other's existence. Now, after 14 years, the twins are accidentally reunited when they are sent to the same summer camp.
They take an initial dislike to each other (Sharon is a proper Bostonian while Susan is a rowdy Californian), but before long they discover their relationship, and as the summer progresses they become close friends.
They decide to get their parents together again, and when camp is over, they switch places in order that each may meet the parent she has never known. Neither parent is aware of the deception until Sharon learns their father is planning to marry a conniving gold digger.
In a feat of early 60's movie magic, Hayley Mills (POLYANNA, THAT DARN CAT) stars as both Sharon and Susan.
Director David Swift. 129 minutes. Not rated, from 1961.