A willful, disorganized teenage girl, Annabelle Andrews, awakens one Friday morning to find herself in the body of her mother, with whom she argued the previous night.
Suddenly in charge of taking care of the New York family's affairs and her younger brother Ben (whom Annabelle has not-so-affectionately nicknamed "Ape Face"), and growing increasingly worried about the disappearance of "Annabelle", who appeared to be herself in the morning but has gone missing after leaving the Andrews' home, she enlists the help of her neighbor and childhood friend, Boris, though without telling him about her identity crisis.
As the day wears on and Annabelle has a series of increasingly bizarre and frustrating misadventures, she becomes gradually more appreciative of how difficult her mother's life is, and learns, to her surprise, that Ben idolizes her, and Boris is actually named Morris, but has a problem with chronic congestion (at least around Annabelle) leading him to nasally pronounce ms and ns as bs and ds. The novel races towards its climax and Ben also disappears, apparently having gone off with a pretty girl whom Boris did not recognize, but Ben appeared to trust without hesitation.
In the climax and dénouement, Annabelle becomes overwhelmed by the difficulties of her situation, apparent disappearance of her mother, loss of the children, and the question of how her odd situation came about and when/whether it will be resolved. Finally, it is revealed that Annabelle's mother herself caused them to switch bodies through some unspecified means, and the mysterious girl who took Ben was Mrs. Andrews in Annabelle's body (to which she is restored) made much more attractive by a makeover Mrs. Andrews gave the body while using it, including the removal of Annabelle's braces, an appointment Annabelle had forgotten about (and would have missed, had she been the one in her body that day).
The book (and especially the film adaptations and its second sequel, Summer Switch) might be considered a modern retelling of Vice Versa, the 1882 novel by F. Anstey, in which the protagonists are a father and son.
Annabelle Andrews 
Disorganized (perhaps even slovenly), rebellious, and something of a tomboy, 13 year-old Annabelle is a fairly typical teenage girl who believes that adults have it easy and quickly finds herself out of her depth when faced with real adult responsibilities and problems. She wants to be free because of her mother bossing her around and is really serious about doing some things she wants to do, but she can not because of her mother.
Ben Andrews, a.k.a. Ape Face 
A typical pre-adolescent younger sibling, 6-year-old Ben delights in creating conflict with Annabelle, torturing her and generally driving her crazy, which has led to her nicknaming him Ape Face. Unknown to Annabelle, he and his friends think she is the epitome of cool, and he wears the nickname as a badge of honor. The truth comes out when she slips up and calls him Ape Face while pretending to be Mrs. Andrews. After she apologizes, he admits that he likes the name – but desperately wants to keep this secret from Annabelle.
A childhood playmate of Annabelle, he has been somewhat afraid of her since an incident in which she hit him in the face with a shovel. Deciding she played too rough, he and his mother have made a point of him avoiding Annabelle since. He is a cheerful and good-looking guy, and does his best to help "Mrs. Andrews" (really Annabelle) as she struggles through a tough day. He has some occasional breathing trouble, attributed to his adenoids and possibly an allergy to Annabelle, which renders his voice nasal, making him unable to pronounce the letters m and n clearly. Because of this, Annabelle has always believed his name to be Boris, when it is in fact Morris. Unfortunately, she suffers the exact opposite misunderstanding when he offers to prepare a "beetloaf" for a dinner party with the limited assortment of culinary ingredients available, and she believes he is going to save dinner by providing a much more sensible meatloaf, but she is won over when she samples the new dish and finds it tasty.
Ellen Jean Benjamin Andrews 
Despite setting the events in motion and later ending them deus ex machina-style, Mrs. Andrews remains (along with Annabelle's body, which she has taken) an unseen character for much of the book, only revealing herself and what she has done at the end of the day.
Three sequels followed Mary Rogers's book, featuring some of the same characters and/or situations. In A Billion for Boris, Annabelle and her friend Morris/Boris discover a TV set that tunes into future broadcasts and begin betting on horse races. The book Summer Switch concerns Ben and his and Annabelle's father, Bill, who inadvertently switch bodies as both are leaving for the summer, leaving the youngster to negotiate Hollywood and Dad to deal with summer camp. Finally, a spiritual sequel about a student and teacher, Freaky Monday, co-written with Heather Hach (screenwriter for the 2003 film), was published in May 2009.
Film adaptations 
A major difference between the novel and the films is the presence of an outside influence switching the often-bickering mother and daughter against both their wills. Because of this, both serve as protagonists, and the films start shortly before the switch, illustrating the conflict between the two, then follow the trouble both have adjusting to their new circumstances, and the new respect and understanding they both come to have for each other before switching back. Mary Rodgers also added a hobby for Annabelle to the story, with an important competition – for which her mother lacks the skill – serving to bring the action to a climax. This addition was kept in subsequent versions, although the original hobby of waterskiing changed to diving and then rock music, and an important simultaneous event for the mother (her wedding rehearsal dinner) was added to the most recent film. The latter two also omit the character of Morris, although the 1976 actor and character have a cameo in 2003, and there is a similar character named Jake in 2003.
The adaptations are:
- The original 1976 version, written by Rodgers, which stars Barbara Harris and Jodie Foster as Ellen and Annabelle.
- A Disney television version was made in 1995, starring Shelley Long and Gaby Hoffmann as Ellen and Annabelle Andrews, switched by magical amulets responding to their wish to have each other's lives.
- Their counterparts in the 2003 remake, co-written by Hach, are Jamie Lee Curtis and Lindsay Lohan. The protagonists' names are changed to Tess Coleman and Anna Coleman. They are switched by magical fortune cookies given to them by a meddling old Chinese woman on Thursday night after she overhears them arguing at her daughter's restaurant. Ben is renamed Harry, the children's father is deceased, and Tess, a somewhat stuffy psychologist and author, has her father staying with her as he visits in advance of her Saturday wedding to fiancé/literary agent Ryan, played by Mark Harmon.
In addition, a television film of the sequel novel Summer Switch, starring Robert Klein and Scott Schwartz, was made in 1984 as part of the ABC Afterschool Special series. A film of the first sequel called Billions for Boris starring Mary Tanner as Annabelle, Scott Tiler as Boris, and Seth Green as Ape Face was released in 1984.
The plot of the anime film from the anime series Atashin'chi appears to be based on Freaky Friday, in which Mikan and her mother accidentally switch bodies due to their being struck by lightning during a storm.
See also 
Vice Versa, the 1882 novel with a similar premise involving a father and son.
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