The Parent TrapNick and Lizzie met and married each other during an ocean cruise on the QE2. Lizzie gave birth to twin daughters, but the couple divorced and lost contact with each other, each parent raising one of the twins without telling her about her sister. Nick raised Hallie in the Napa Valley and became a wealthy wine grower, while Lizzie raised Annie in London and became a famous wedding gown designer.
After the ocean cruise and onboard wedding ceremony, the story jumps ahead to a summer in which Nick and Lizzie coincidentally enroll their daughters in the same summer camp. Annie and Hallie, who are now eleven years old, first meet at the end of a fencing match, when they remove their masks and see that they look alike. A comical hostility between the two girls leads to a prank war that ends when the camp counselors fall into one of Hallie's traps and isolate the twins from the other girls.
Living together, the girls discover that they were born on the same day and they each have half of a torn wedding photograph of their parents. Realizing with delight that they are twins, the girls hatch a plan to meet their previously unknown parents: Each girl will train her twin to impersonate her, and they will switch places at the end of the summer.
When camp is over, the plan succeeds: Hallie goes to London, where she meets her mother, her grandfather, and the James family's butler Martin (Kunz). Annie goes to California, where she meets her father, the Parker family's housekeeper Chessy (Walter), their dog Sammy, and Nick's fiancée, a young gold digger named Meredith (Hendrix). Distressed by Meredith's deviousness, Annie telephones Hallie and persuades her to bring Lizzie to California to break up the engagement. Meanwhile, the girls' identities are discovered, and, with the exception of Nick, who remains unaware of the switch, their newfound family members tearfully welcome them.
In order to bring Nick and Lizzie together, Annie, Hallie, Grandpa, Chessy, and Martin conspire to have them meet at a hotel by arranging for Nick to meet Meredith's parents and by not telling Lizzie about Meredith. Nervous about meeting Nick, Lizzie asks Martin to accompany her and Hallie. After a few comical mixups in the hotel, Nick and Lizzie see each other, Nick finally learns about the switch, and Nick and Lizzie are told the real purpose of their visit: With the help of Martin, Chessy, and Grandpa, the twins have arranged a candlelit dinner for Nick and Lizzie on a yacht decorated to recreate their first meeting. At dinner, Lizzie mentions that Nick didn't follow her after she left him. They make plans for the twins to spend holidays together, but decide against resuming their relationship.
Hallie and Annie dislike this idea, so they force their parents to take them camping by refusing to reveal which twin is which. After Lizzie persuades Nick and the girls to take Meredith instead of herself, the twins play tricks on Meredith, who insists that Nick choose between her and his daughters. Nick chooses the twins, and Meredith breaks off the engagement.
After Meredith leaves, Nick shows Lizzie his wine collection, which includes the wine they drank at their wedding. Lizzie is touched by this gesture at first, but has a change of heart and returns to London with Annie. However, when Lizzie and Annie get home, they find Hallie and Nick waiting for them, having flown there on the Concorde. Lizzie is fearful of remarrying, but she yields to Nick's confidence, and Annie and Hallie look on happily as Nick and Lizzie embrace. The final credits feature photographs of Nick and Lizzie's second wedding, also aboard the QE2, with the twins as bridesmaids and Martin presenting Chessy with an engagement ring.
