Sharon McKendrick (Hayley Mills) arrives at Camp Inch from Boston, Massachusetts. It isn't long before she encounters Susan Evers (Hayley Mills), whom Sharon's camp-mates notice seems to resemble her.An insult by Susan soon leads to a rivalry between the two girls, ending with a fight during a dance at the camp. The head of the camp Miss Inch (Ruth McDevitt), then isolates the two in a cabin for the remaining weeks as punishment.At first, the two girls refuse to talk to each other, but eventually, they open up and begin to discuss their lives. Sharon tells of living in Boston with her mother, and Susan tells of living on her father's ranch in Monterey, California. Both note that neither of their parents are married, but when It is revealed that they both share the same Birthday (November 12th), Sharon suspects something. Susan mentions that she once saw a picture of her mother, before her Dad found out and hid it away. Sharon then reveals a picture of her own mother...and the two then realize that they are sisters!As they realize that neither of their parents has remarried, Sharon feels that they must still love each other, leading to Susan hatching a bold plan: they will switch places once camp is over, and live in each other's respective shoes, getting to know the parent they've never met. There is also the hope that if the switch is found out, their parents will have to "unswitch" them, with the girls hoping they can bring their divorced parents back together.The two then learn the basics of each other's lives, and Susan completes the transformation by cutting Sharon's hair, and getting her to bite her fingernails (one of Susan's habits).Camp eventually ends, and the two sisters part ways.Susan heads to Boston, where she meets her Grandfather, Grandmother, and her Mother, Maggie (Maureen O'Hara). Susan is eager to know more about her Mother's former relationship, and after making her fear that she's engaged in a serious summer romance, the two go out for a walk.Susan quickly puts her Mother's fears to rest about boys, and gets her to open up about her first date with her Father. Her mother tells of an Italian restaurant called Martinelli's, as well as the melody to a song that played during the date.Meanwhile, Sharon arrives in California, and meets her Father, Mitch (Brian Keith). Things get a little awkward when Sharon's accent pops out a few times, and upon arriving at the ranch, Susan's dog Andromeda does not greet her warmly.As the housekeeper Verbena (Una Merkel) helps her unpack, Sharon sees her father talking to a woman in the house's main courtyard. Verbena nonchalantly tells that Mitch has been seeing a woman named Vicky (Joanna Barnes) for a few weeks, but Verbena feels this woman is just after Mitch's money.Sharon is soon introduced to Vicky, and while Mitch gets them some drinks, Sharon tries to convince Vicky that her father is a real ladies man, not looking to settle for just one woman...leading Vicky to believe that Mitch's daughter could be trouble for her plans.Before they parted, the twins scheduled an early-morning call after they returned to their homes. During the call, Sharon demands that Susan get their mother to California after meeting Vicky, but Susan stubbornly refuses, claiming she wants to spend more time with their Mother, leaving Sharon to deal with Vicky on her own.Following the call, Sharon accompanies her Father to the local country club, and attempts to get him to tell her more about her Mother, but he misconstrues her questioning. The mood is broken when the two meet Vicky and her mother Edna (Linda Watkins) afterwards, before Father and Daughter return to the ranch.Verbena finds Sharon talking to herself, and questions her about some of the changes she's noticed. It is when she begins to claim that the young girl seems to be a different person, that Sharon quietly confesses who she really is, and swears Verbena to secrecy.Mitch continues to try and discuss his plans to wed Vicky, but Sharon tries to play dumb and deter him, eventually getting exasperated and throwing a tantrum, leading her father to walk away.Vicky attempts to once again charm Sharon, but the girl is having none of it. When Sharon claims Vicky may just be marrying Mitch for his money, Vicky gets defensive, and declares that she is going to marry Sharon's father, whether the girl likes it or not.Realizing the direness of the situation, Sharon sends Susan a cryptic telegram, requesting another early-morning call.During the call, Sharon explains to Susan that the plans for the wedding are moving quickly, and they need to get their mother out to California right away. Susan says she will explain herself to their Mother the next day, but on her way back to her room, Susan encounters her Grandfather, who overheard the whole conversation and now knows the truth.The next day, Susan confesses who she is, and explains to her mother about Mitch's impending marriage. Maggie makes plans to return Susan to her ex-husband...right after she and her daughter stop off in New York City for some clothes shopping.When Susan and her mother arrive at Mitch's ranch, it happens to be on the day when Vicky, her mother, and Reverend Mosby (Leo G Carroll) have stopped by to discuss the impending wedding. Naturally, the new arrivals throw the plans into major disarray, and the planning session is called off.Later on that evening, Mitch and Maggie are directed to the main area of the house, where Verbena has prepared an Italian meal, and the ranch hand Hecky (Crahan Denton) provides atmosphere as a gypsy. The parents are then surprised when their daughters perform a musical duet, before claiming they are going to bed.Mitch is unsure what is going on, but when Maggie finds a sign mentioning "Martinelli's," she realizes the girls are trying to recreate their first date. However, the conversation soon turns into an argument, and it seems the girls' plans have fallen through.The next day, Maggie prepares to return to Boston with Sharon, only for the girls to appear wearing identical clothing. Neither girl is willing to tell who is Sharon or Susan, and propose a deal: they want Maggie and Sharon to go on the camping trip Mitch and Susan are going off on that afternoon, with the caveat that when they get back, they will reveal which girl is which.The two adults agree to the arrangement, but when Vicky comes over and finds out about the arrangement, Maggie decides to make a compromise: she'll stay behind, and Vicky can go with Mitch and the girls to get to know them.Naturally, Vicky isn't accustomed to roughing it. The girls play pranks on her, from putting a lizard on her canteen, to claiming there's a way to keep mountain lions away. When they reach the campsite, the girls trick her into thinking the nearby river is shallow, and she ends up soaking wet.During dinner, Vicky refuses to eat the trout that's been caught, and is also informed by Mitch that the bug spray the girls gave her, is just sugar and water.After Vicky goes to sleep, the girls rig her tent, including pouring honey on her feet that attracts a few bear cubs. This is the final straw for Vicky, who claims that being with Mitch "isn't worth it," and demands the ranch hand Hecky "get her out of the stinking fresh air."After this, Mitch and the girls return back to the ranch. The girls go to their room, and finding Maggie has sent Verbena away for the day, Mitch dresses up for a dinner with his ex-wife.Over the meal Maggie has prepared, they seem to work through their differences.Some time later, the twins are in bed, when one awakens, and tells her sister that she had a dream, of their parents getting remarried at the ranch.