n 1986, Nick Parker (Dennis Quaid) and Elizabeth James (Natasha Richardson) meet and get married during an ocean cruise on the RMS Queen Elizabeth 2. After the birth of their twin daughters, Annie and Hallie, Nick and Elizabeth divorce and lose contact, each parent raising one of the twins without telling her about her sister. Nick raised Hallie in the Napa Valley and he became a wealthy wine grower, while Elizabeth raised Annie in London and became a famous wedding gown designer.
Eleven years later, in 1997, Nick and Elizabeth coincidentally enroll their daughters in the same summer camp in Maine called Camp Walden. Hallie and Annie 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 in an isolation cabin, Hallie and Annie 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 camp.
When camp is over, the plan succeeds. Hallie, pretending to be Annie, goes to London, where she meets her mother, her grandfather; Charles, and the James family's butler; Martin (Simon Kunz). Annie, pretending to be Hallie, goes to California, where she meets her father, the Parker family's housekeeper; Chessy (Lisa Ann Walter), their dog; Sammy, and Nick's young, opportunistic fiancée; Meredith Blake (Elaine Hendrix), who is only interested in Nick's money. Distressed by Meredith's deviousness, Annie telephones Hallie and persuades her to bring Elizabeth to California to break up the engagement. Soon, everyone except for Nick and Meredith, who remain unaware of the switch until their newfound family members surprise them, discovers the girls’ identities.
In order to bring Nick and Elizabeth together, Hallie, Annie, Chessy, Martin, and Charles conspire to have them meet at a hotel in San Francisco by arranging for Nick to meet Meredith's parents and by not telling Elizabeth about Meredith. Nervous about meeting Nick, Elizabeth asks Martin to accompany her and Hallie. After a few comical mix-ups in the hotel, Nick and Elizabeth see each other, Nick finally learns about the switch and the girls host a candlelit dinner for Nick and Elizabeth, served by Chessy and Martin, on a yacht decorated to recreate their first meeting. At dinner, Elizabeth mentions that Nick did not follow her after she left him, and Nick responds that he was not sure if Elizabeth wanted him to. They make plans for the girls to spend holidays together, but decide against resuming their relationship.
Hallie and Annie dislike this idea, so they force their parents to take them on Hallie and Nick's annual camping trip by refusing to reveal which twin is which. After Elizabeth persuades Nick and the girls to take Meredith instead of herself, the girls play tricks on Meredith. Annie puts rocks in her bag when she is tired, and puts a lizard on her Evian bottle, which she later puts on her head. The twins also put sugar and water together in Meredith's mosquito repellant bottle, and then they take her floatable mattress out the tent and put it in the lake. Meredith becomes enraged and gives Nick an ultimatum that he chooses her or the girls. Nick chooses the girls over Meredith and breaks off the engagement.
After Meredith leaves, Nick shows Elizabeth his wine collection, which includes the wine they drank at their wedding. Elizabeth is touched by this gesture at first, but has a change of heart and returns to London with Annie. However, when Annie and Elizabeth get home, they find Hallie and Nick waiting for them, having flown there on the Concorde. Elizabeth is fearful of remarrying, but she yields to Nick's confidence, and Hallie and Annie look on happily as Nick and Elizabeth embrace. A year later, Nick and Elizabeth are remarried aboard the QE2, with the girls as bridesmaids and Martin presenting Chessy with an engagement ring.