The film starts as Tess Coleman (Jamie Lee Curtis) is getting ready to record another episode of her therapy podcast. Ever since she strengthened her relationship with her daughter Anna (Lindsay Lohan), Tess has now helped Anna raise her teenage daughter Harper (Julia Butters), whom Anna gave up her musical ambitions for. Anna tries to get Harper ready for school, but the teenager is out surfing with her friends. She manages to get back home in time for Anna to take her to school.
At school, Harper is annoyed by a new snobby British girl named Lily Reyes (Sophia Hammons). During chemistry class, Harper is forced to partner up with Lily, who just doodles her fashion ideas into her notebook. The two argue and mix the wrong chemicals together, causing a foam explosion in the classroom.
Anna is called to the school for the incident. She runs into Lily’s father Eric (Manny Jacinto), who is handsome and charming. They quickly form their own chemistry (no pun intended) and start dating.
Over six months later, Anna and Eric are engaged to be married, to the dismay of Harper and Lily. Making matters more complicated is that Anna and Eric plan to move to London, as Eric knows that Lily has not gotten over the death of her mother. Harper is not happy about potentially uprooting her life and leaving her friends, in addition to having to become stepsisters with Lily. Lily also doesn’t want to become part of the Coleman family.
Anna goes to her job as a talent manager. Her client, pop star Ella (Maitreyi Ramakrishnan), is in shambles because her boyfriend, fellow artist Trevor Travis, has broken up with her and moved on with another girl, even making a song about his relationship with Ella. Ella also happens to have been a fan of Anna’s old band Pink Slip.
Harper and Lily are participating in a bake sale. Harper hides Lily’s organic dessert options, prompting Lily to throw out Harper’s cookies. She responds by smearing Lily’s dessert on her clothes, and Lily puts a cupcake in Harper’s face. The students then engage in a food fight, which ends when someone throws key lime pie in the face of Principal Waldman (X Mayo). She gives all the students in-school suspension, making everyone hate Lily and Harper. The girls get chewed out at home by Anna once she finds out about the incident.
Everyone later goes to Anna’s bachelorette party that night. Among the guests are Anna’s former band mates Maddie (Christina Vidal Mitchell) and Peg (Haley Hudson), as well as Pei Pei (Rosalind Chao) and her mother (Lucille Soong). Tess tries to talk to Lily about her reservations with joining the family, but Lily is rudely dismissive. Likewise, Harper vents about her frustrations with the wedding to Anna. Anna and Tess are met by a wannabe psychic, Madame Jen (Vanessa Bayer). She gives them a free reading and deduces that they have “crossed paths” in life before, and that they learned a lesson that may serve them well again. The mother and daughter get creeped out and leave, initially accusing Pei Pei of telling Jen the story about when they switched bodies. Later, Madame Jen finds Harper and Lily and repeats a mantra to them – “Change the hearts you know are wrong, so that you may be where you belong.” The room then begins to shake, but only Anna, Tess, Harper, and Lily feel it.
The following morning, the ladies all discover that they have switched bodies. Anna and Harper have switched places while Tess and Lily have traded bodies. The girls are horrified while Anna and Tess try to calm them down and figure out how to undo the curse this time. Harper and Lily must handle Anna and Tess’s job, and they choose to dress up in their own signature styles, to Anna and Tess’s chagrin. Meanwhile, they must endure Harper and Lily’s detention in school. The girls make a secret plan to break Anna and Eric up so that they don’t have to deal with each other anymore.
Lily goes to a photo shoot with Ryan (Mark Harmon) since Tess was scheduled to have author photos taken for her upcoming book. Lily uses some kind of lip balm but Tess’s face has an allergic reaction to it, making her lips puff up like a fish. She then joins Ryan in a pickleball match against two aggressive players, Veronica (June Diane Raphael) and Vivian (Mary Sohn). Despite Lily’s nervousness in playing, she succeeds in helping Ryan win the match.
Anna and Tess find that the students’ detention advisor is none other than Elton Bates (Stephen Tobolowsky), Anna’s former teacher who tormented her because he was bitter that Tess rejected him when they were students. Anna and Tess try to utilize a rebellious attitude towards Bates, but it only gets them and everyone else sent to do laps around the track and to clean under the bleachers, making the students hate them (Harper and Lily) even more.
Harper goes to a photo shoot for Ella that Anna was supposed to oversee, but Ella is still depressed over Trevor. She gets Lily to join them since she is a fan of Ella, and Lily is able to give her a pep talk that encourages her to take control of her future. Lily also gets a chance to play with certain designs as Ella agrees to continue the shoot. Ella also asks Harper (thinking it’s Anna) about an unreleased song that she (Anna) wrote called “Baby”, which talks lovingly about an unknown person. Harper and Lily think that Anna wrote the song about her former boyfriend Jake (Chad Michael Murray) and that she still pines for him. The girls then conspire to get Anna back together with Jake so they can break up the marriage.
Anna and Tess swipe Bates’s phone from him since he took the students’ phones, and they are annoyed to see Harper and Lily going on a joyride with Ella. The two leave the school on motor scooters, but they take a moment to enjoy being teenagers again and go on a junk food binge. They then go to a Starbucks and find Madame Jen to ask how to undo the spell this time, and she just repeats the same mantra she told Harper and Lily.
Harper and Lily go to Jake’s record shop while dressed in “sexy” clothing so that Harper can seduce Jake. He gives Harper some old records that Anna was going to sign and donate. Harper then attempts to awkward flirting and seduction techniques courtesy of Lily, but it just makes Jake think she is having a stroke. Lily then admits to Harper that she doesn’t have experience with boys as she previously said she did. Jake sees Lily as Tess and is still clearly infatuated with Tess. Harper then remembers Anna needs to go to an immigration meeting with Eric, so they borrow Jake’s car to go there.
At the immigration meeting, Harper tries to put Eric off and make him think they aren’t right for each other. During the interview, Eric makes it clear he knows enough about Anna and that he really does care for her. The immigration officer can see that Anna and Eric are right for each other and gives them an approval. Harper and Lily then go to the beach, where they bond and admit they are starting to hate each other less.
The girls go home and are chastised by Anna and Tess, but they realize they have no authority over the girls while being in their bodies.
The next morning, Harper goes surfing in her mom’s body and is joined by Eric. He tells her that he doesn’t want to move back to London just because he misses his late wife, even if it’s what Lily wanted. Seeing how much Eric cares for Anna causes Harper to reconsider her plans.
Later that day, everyone gathers for the rehearsal dinner. Lily calls Jake and invites him to show up. She also learns that Eric is no longer planning to go back to London, which infuriates her. After Jake arrives, things get awkward when Lily (as Tess) declares that she doesn’t want Anna and Eric to marry. Eric decides that while he does want to marry Anna, he doesn’t want to make a choice between her and Lily, so he calls off the wedding. Anna is heartbroken and disappointed with Harper. Anna leaves because she is getting urgent texts from Ella as she is about to perform. Harper follows and tries to apologize.
Tess talks to Ryan, who sees Lily as Tess crying. He expresses to Tess as Lily that he worries about Tess being alone if Anna and Harper were to move to London, making Tess see how much he worries about her. Tess then talks to Lily about how hard it was for Anna to accept her marriage to Ryan, but they made it work.
Lily runs after Eric in the street, trying to apologize for not wanting the wedding to go through. He does still want to marry Anna but reiterates that Lily is the most important person to him and that he can’t compromise her life to do that. Tess as Lily comes out to give Eric her blessing, and the three head to the venue where Ella is performing.
At the venue, Anna and Harper see that nothing is wrong with Ella and that she surprised Anna by bringing Maddie and Peg to the show so that Pink Slip can perform. Anna and Harper then talk about the song, since Harper thought it was about Jake. Anna clarifies that the song is about Harper herself and how much Anna loves her. Harper steps up as Anna to perform while Anna opts to play for her in the background. Before people can see how bad it will look, with Harper poorly miming the guitar chords, she stops to introduce Anna as Harper to the audience so that they can perform together. Anna plays the guitar while they both sing “Baby” together. Eric, Tess, and Lily arrive and see the show. After witnessing the display from Anna and Harper, Lily is emotionally moved and accepts becoming part of a new family. The earth shakes once again, and the ladies all return to their regular bodies. Anna is able to perform with Maddie and Peg, while Eric is crowd-surfed to the stage so he and Anna can reconcile and kiss. Meanwhile, Harper and Lily accept each other as sisters.
The wedding goes ahead as planned, with Anna and Eric getting married, and Ella officiating for them.
During the credits, the ladies go to a book-signing event for Tess. She is then mortified to see the photo they picked with her puffed up lips, causing Harper and Lily to make a run for it. This is followed by outtakes and bloopers.
Anna and Tess Coleman have since strengthened their relationship as mother and daughter, while also taking care of Anna’s teenage daughter Harper. Anna starts dating a British man named Eric Reyes and gets engaged to him, which dismays Harper because she does not get along with Eric’s snobby daughter Lily. Making things more complicated is that Anna and Eric plan to move to London after the wedding, which upsets Harper and Lily since they refuse to be stepsisters.
At Anna’s bachelorette party, the four ladies meet Madame Jen, a wannabe psychic who gives Anna and Tess a reading, and then gives Harper and Lily a mantra for them to learn a lesson. The next day, Anna and Harper have switched bodies, while Lily and Tess have switched. Anna and Tess endure the girls’ detention and enjoy having teenage bodies again, while Harper and Lily plan to use their adult bodies to break up the wedding.
Harper and Lily try to get Anna back with her ex-boyfriend Jake after hearing an unreleased song from Anna, which doesn’t work. Harper starts to see that Eric genuinely loves her mother and also doesn’t want to love to London, which she doesn’t tell Lily about since they have also genuinely started to bond. However, things blow up at the rehearsal dinner when Lily invites Jake to show up, and then uses Tess’s body to object to the wedding. This causes Eric to call off the wedding, and Anna is disappointed in Harper.
After realizing how wrong she was, Lily apologizes to Tess and tries to convince Eric that they both want the wedding to proceed. They go to a venue where Anna’s client Ella is performing, and she has invited Anna’s old Pink slip band mates Maddie and Peg to perform. Anna tells Harper that her song wasn’t about Jake, but about Harper. The two perform as mother and daughter, while Lily sees and accepts her new family. This allows the ladies to switch back, and Anna and Eric reconcile.
The wedding goes ahead as planned, and Harper and Lily accept each other as sisters.
