HALLIE: So if your mom is my mom and my dad is your dad ... and we're both born on October 11th, then you and I are ... like ... sisters.
ANNIE: Sisters?!? Hallie, we're like ... twins!!!
Hallie Parker lives an idyllic life spending her days swimming and riding horses with her loving and attentive father and sharing secrets and jokes with her warmhearted nanny, Chessy (LISA ANN WALTER). Everything is perfect ... except as Hallie says, "I'm almost a teenager and I'll be the only girl I know without a Mom to fight with."
Annie James has an exciting and sophisticated life, growing up in a London townhouse with a beautiful and loving mother, a benevolent grandfather (RONNIE STEVENS), and a classically-trained yet slightly off-kilter butler named Martin (SIMON KUNZ). Everything is perfect ... except as Annie says, "A dad is an irreplaceable person in a girl's life."
Destiny comes into play when both girls arrive in Maine at Camp Walden for Girls. Instantly distrustful of each other, the girls are punished by camp owners Marva Sr. (POLLY HOLLIDAY) and Marva Jr. (MAGGIE WHEELER) for their hijinks and sentenced to spend the rest of the summer as the sole inhabitants of the dreaded "isolation cabin." It is there that the girls realize they are indeed twins. Intrigued by what caused their parents to split up a decade earlier and each determined to meet their "other" parent, the girls decide to take matters into their own hands and switch places.
With Annie's accent and mannerisms down pat, Hallie is off on a London adventure, that includes a tour of the city and a chance meeting with supermodel Vendela. At the same time, Annie is off to Napa Valley where she is enamored with the California lifestyle, but is shocked to discover a wrinkle in the plan to get their parents back together. Upon arriving home, Annie meets Meredith Blake, a slinky 26-year-old beauty who calls her dad 'Nicky.' Trouble is afoot and Annie 911's Hallie in London asking for help.
In London, Hallie admits her true identity and joins her mother and Martin on a trip to San Francisco under the guise of switching the sisters back. Nervous at the prospect of seeing her gorgeous and charming ex after eleven years, Elizabeth discovers the complimentary in-flight cocktail bottles and after a nine-hour flight, arrives in San Francisco ready for anything.
Meanwhile, Nick, Chessy and Annie simultaneously arrive at the hotel in San Francisco to rendezvous with impending in-laws for a pre-nuptial get-together. Mother (JOANNA BARNES) and daughter Meredith are thrilled at the millions ($$$) of possibilities this marriage presents. Unaware that both his daughters and his ex-wife are in the hotel, Nick is in for a comic jolt.
After a futile attempt to rekindle their parents' romance with a candle-lit dinner on a yacht, the time comes for Elizabeth and Annie to return to England. However, the twins refuse to reveal their true identities and strike a deal that takes the entire family on Nick's annual camping trip.
Elizabeth convinces Meredith to join the outing and then conveniently bows out of the trip. The opportunity to torture Meredith in the great outdoors is not lost on Hallie and Annie. After an array of traumatic encounters orchestrated by the girls, Meredith is driven to give Nick an ultimatum ... it's me or them ...
