A few days before 16-year-old Lara Jean’s big sister, Margot, leaves for her first year of college in Scotland, Margot tells Lara Jean that she broke up with her long-term boyfriend, Josh. Lara Jean can’t believe it—Josh and Margot belong together, and to make matters worse, Lara Jean used to be in love with Josh. But she keeps Margot’s secret over the next few days as she, Daddy, and her nine-year-old sister Kitty help Margot get ready to go.
Margot has been taking care of Daddy and her younger sisters since Mommy died six years ago. With Margot gone, Lara Jean is going to have to become more responsible—and Lara Jean is nervous about that. To deal with her feelings about Josh, Lara Jean pulls down the hatbox Mommy gave her. In it are five letters she wrote to five boys as an exercise to help herself stop loving them. She adds a postscript to her letter to Josh, since she realizes she’s still in love with him.
Mere hours after Margot gets on her plane for Scotland, Lara Jean, who’s a nervous driver, gets in a car accident. A former friend and current classmate, Peter Kavinsky, stops to help her, but Lara Jean waves him on—he’s going to see his girlfriend, Genevieve, who is popular, frightening, and controlling. Josh comes to rescue Lara Jean. Later, as Josh hangs out with Lara Jean and Kitty, Lara Jean teases Kitty: Kitty wants a dog, and Lara Jean knows Kitty would like to name the dog Josh since she also has a crush on Josh. Though Lara Jean doesn’t spill Kitty’s secret, Kitty gives her the silent treatment for days in retaliation for teasing her.
Things turn upside-down a few days after school starts. Josh admits that Lara Jean was his first crush, rumors circulate about Peter and Genevieve breaking up, and Peter approaches Lara Jean during gym—he got the letter she wrote to him years ago. Lara Jean is distraught. Her hatbox, and the five letters in it, are gone. When Josh knocks on the door that night and asks to speak to Lara Jean, she hides. But at school the next day, when Josh tries to confront her again about her letter, Lara Jean lies that she has a boyfriend, leaps into Peter’s arms, and kisses him. Over the next week, Lara Jean and Peter come up with a plan. They’re going to pretend to date each other to throw Josh off and to make Genevieve jealous. They come up with rules and draw up a contract. Since neither of them has a crush on the other, they believe this will be easy.
Since Lara Jean’s car is still being repaired after the accident, Peter starts to drive her to school. He shows up at her house the first day in his two-seater Audi, but when Lara Jean begs him to drop Kitty off as well, he starts driving a van. Over the next week or so, the letter Lara Jean wrote to Kenny from camp gets returned unopened, and Lucas Krapf gives his letter back to Lara Jean—he’s flattered, but he’s gay. He encourages her in her relationship with Peter. Lara Jean doesn’t tell Margot about dating Peter. She keeps a close eye on Genevieve, waiting for Genevieve to do or say something nasty. Nothing happens.
One of the requirements of Lara Jean and Peter’s pretend relationship is that Peter has to write Lara Jean a note every day. But when Peter shows up at Lara Jean’s house one Friday night, ready to take her to a football game and then a party, Lara Jean has to admit she didn’t read the note inviting her out. And she’s in the middle of making six-dozen cupcakes for the PTA bake sale at Kitty’s school. When Daddy gets home and meets Peter, he insists on finishing the cupcakes and tells Lara Jean to go to the party. As Lara Jean searches through Margot’s clothes for a sweater, though, she finds a letter to Margot from Josh. From it, she learns that Margot and Josh broke up because they had sex. Even though Lara Jean’s best friend Chris insisted they probably had slept together, Lara Jean can’t believe it—she and Margot had a pact to wait. She doesn’t know Margot at all.
The party is stressful. Lara Jean is pressured into telling Genevieve that she and Peter got together the week before school started, which is the story that they agreed on, but Genevieve is clearly upset. Lucas points out to Lara Jean that Peter and Genevieve obviously care for each other still, since they fight at the party. In the car on the way home, Peter says he wishes Genevieve didn’t have such a hold on him.
The next weekend, Lara Jean has a great time accompanying Peter to an estate sale (his mom, Mrs. Kavinsky, owns an antique store and wants antiques from the sale). She also has dinner with Peter’s family. But Lara Jean starts to feel less secure in her relationship with Peter as he continues to seek out Genevieve. To make matters worse, Josh doesn’t like Peter and insists that Lara Jean could do better. The rivalry between the boys escalates on Halloween, when Lara Jean dresses as Cho Chang from Harry Potter. She and Josh didn’t coordinate, but he dresses as Harry Potter—which makes Peter, who dresses as Superman, angry.
During a virtual dinner with Margot, Kitty spills the beans that Lara Jean is dating Peter. Margot is aghast; Peter cheated on a test in middle school and isn’t smart, according to her. Lara Jean wishes Margot could just be happy for her.
Lara Jean continues to find herself caught between Josh and Peter. She agrees to go with Josh to the diner, where they used to go with Margot—but Peter is angry that Lara Jean and Josh hung out in public, and Josh continues to say that Peter is “douchey” and will take advantage of Lara Jean. Lara Jean finds herself liking Peter more and more, so she lets him convince her to sign up for the school ski trip. She’s nervous, though, when she hears that lots of kids have sex and skinny dip in the hot tub on the ski trip. She convinces Chris to come with her, both for emotional support and to annoy Genevieve.
When December first rolls around, Lara Jean, Kitty, and Peter start work on the annual Christmas Cookie Bonanza, where the Coveys make dozens of cookies. They decide to try some new cookies this year, though Josh drops by and begs them to make his favorite cookie. Lara Jean feels odd making cookies without Margot, and she feels even worse about getting the Christmas tree before Margot gets home. But Kitty begs to do the tree early, and Daddy agrees.
Josh helps the family decorate—and while he and Lara Jean are alone, he says that she should’ve told him she had a crush on him years ago. He kisses her. Lara Jean tells Peter about the kiss the next day and Peter is enraged. But when Lara Jean and Josh talk about their kiss, Lara Jean has an epiphany: she’s not in love with Josh anymore. Deciding that she’s accomplished her goal of getting over Josh, Lara Jean tries to break up with Peter, but he convinces her to keep up the charade until after the ski trip.
Margot flies home the next day. She’s clearly upset that her family started Christmas Cookie Bonanza and put up the tree without her, and she says mean things about Peter. To make things worse, Margot decides the family should throw their first recital party since Mommy died, an event that Lara Jean always hated as a kid. Margot seems unwilling to accept that both Lara Jean and Kitty have grown up and matured since she left, which makes Lara Jean angry.
Finally, it’s the day of the ski trip. Lara Jean insists on sitting with Chris on the bus instead of Peter, since she figures that Peter and Genevieve are going to get back together on the trip. But this makes Peter angry. He ignores her all day at the lodge until, finally, Lara Jean confronts him. They argue—and Peter admits he’s fallen for Lara Jean over the course of their pretend relationship. Lara Jean’s first reaction is to lash out in fear and anger, but then she realizes that she likes Peter too. She goes to talk to him in the hot tub, confesses her feelings for him, and they kiss. Lara Jean is ecstatic—until the bus ride home the next morning, when Genevieve shares that Peter is telling people that he and Lara Jean had sex in the hot tub. It’s clear that Genevieve is the one spreading the rumor, but Peter doesn’t do much to shut it down. Lara Jean refuses to talk to him.
Once she’s back home, Lara Jean she deletes Peter’s number and throws away all the notes he wrote to her. When Kitty gets the puppy she’s been asking for, Lara Jean wishes she were young enough still that a puppy could fix all her problems.
The recital party is set for the day after Christmas. The party seems to be going well until Peter shows up; Kitty apparently invited him. He asks Lara Jean to speak privately, but Josh joins in their conversation. The boys exchange insults and then Peter notes that Josh kissed Lara Jean. Margot overhears and runs to her bedroom, sobbing. She feels like Lara Jean betrayed her.
The next few days are tense at Lara Jean’s house. Finally, Daddy seeks out Lara Jean and says that while he thinks she’s too young to have sex, he made her a doctor’s appointment so she can be safe. Lara Jean knows that Margot told Daddy she’s been having sex to hurt her, so once Daddy leaves, Lara Jean confronts Margot. She reveals that she knows Margot and Josh had sex. The girls scream at each other, but then Lara Jean admits she looks up to Margot and just wants Margot’s approval. Margot admits that she’s upset because it seems like her little sisters don’t need her as much as she thought they did. The girls make up.
On New Year’s Eve, Margot and Daddy go out to parties. Home alone with Lara Jean, Kitty admits that she’s the one who sent Lara Jean’s letters months ago and stole her hatbox. She gives Lara Jean the hatbox back and inside, Lara Jean discovers all the notes from Peter that she tried to throw away; Kitty pulled them out of the trash and saved them. Reading them, Lara Jean feels like they are a love letter, and so after she and Kitty light sparklers at midnight, she starts to write Peter a love letter in return.
