In 1986, Nick Parker (Dennis Quaid) and Elizabeth James (Natasha Richardson) meet and get married during an ocean cruise on the RMS Queen Elizabeth II. After the birth of their twin daughters, Annie and Hallie (Lindsay Lohan), Nick and Elizabeth divorce and lose contact, each parent raising one of the twins without telling her about her sister. Nick raises Hallie in Napa Valley and becomes a wealthy wine grower, while Elizabeth raises Annie in London and becomes a famous wedding gown designer.
Over 11 years later, Nick and Elizabeth coincidentally enroll their daughters at the same all-girls 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 exactly 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 they were born on the same day, and each has half of a torn wedding photograph of their parents. Realizing with delight they are twins, the girls act a plan to meet their previously unknown parents. Each girl trains the other to impersonate her, with the intent to switch places at the end of summer camp.
When camp is over, the twins put their plan into action. Hallie, pretending to be Annie, goes to London to meet her mother, her maternal grandfather, Charles, and the James family's butler, Martin (Simon Kunz). Annie, pretending to be Hallie, goes to California to meet 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. However, Charles catches Hallie on the phone, and in California, Chessy figures out that it's Annie and not Hallie there. Soon, everyone except for Nick and Meredith, who remain unaware of the switch until their newfound family members surprise them, discovers the girls' identities.
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 Hallie and her. After a few comical mix-ups in the hotel, Nick and Elizabeth eventually see each other. Nick finally learns about the switch and the girls host a candlelit dinner for their parents, 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.
Dismayed, Hallie and Annie force their parents to take them on a camping tripthe annual outing Nick and Hallie take before school startswhile keeping quiet about which twin is which. Elizabeth coaxes Meredith into camping with Nick and the girls in her place, while she stays behind at the Parker residence. During the trip, the girls play various tricks on Meredith. It all boils over after the two execute their final prank, of sending Meredith out on the lake by their camp while she is sleeping. Meredith becomes enraged and gives Nick an ultimatum: her or his daughters. Nick chooses the girls over Meredith and breaks off the engagement. Although Annie and Hallie are both punished for their shenanigans, they accept it, as they are rid of Meredith for good. Later, out of earshot of the girls, Nick asks Elizabeth to "Remind me to thank them one day."
When Nick returns home, he shows Elizabeth his wine collection, which includes the wine they drank at their wedding. Elizabeth is touched 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. They explain that they had realized they didn't want to lose Annie and Elizabeth again and Nick finally chased after Elizabeth like he should've done the first time she left. Elizabeth is fearful of remarrying, but she yields to Nick's unwavering confidence, and Hallie and Annie look on happily as their parents embrace. The ending shows Nick and Elizabeth getting remarried aboard the QE2, with the girls as bridesmaids and Martin and Chessy becoming engaged to be married.