On New Year’s Day, Lara Jean Covey holds a love letter in her pocket, waiting to send it to Peter Kavinsky. Lara Jean has a complicated past with Peter. At the start of junior year, she began a fake relationship with him so they could both look cool in front of their respective romantic interests.
Somewhere along the way, the fake relationship developed into a genuine one, but miscommunications and hurt feelings made them stop speaking to each other. Now that Lara Jean is certain about her romantic feelings for Peter, she goes to his house to deliver the letter. It turns out that Peter was missing her also, so they decide to begin a real relationship.
Lara Jean goes home to announce the good news to her sisters. However, her 9-year-old sister, Kitty, says that their oldest sister, college freshman Margot, is crying because, unlike Lara Jean, she was unable to reconcile with her ex-boyfriend Josh. Margot and Lara Jean have a long heart-to-heart talk about the events of the past few months, which helps them regain the closeness that they lost when Margot moved to Scotland for college.
Margot mentions that she’s most upset over losing Josh’s friendship, and Lara Jean remembers how she used to be best friends with Peter’s ex-girlfriend Genevieve in middle school. Their friendship dissolved over time, and Gen now resents Lara Jean for dating Peter. She even spreads a false rumor that Lara Jean and Peter had sex in a hot tub on the school ski trip, when they only kissed.
On Lara Jean and Peter’s first date at the movies, Gen shows up at the same movie. Instantly, Lara Jean is plagued with doubts about her relationship with Peter and wonders if he’s still texting Gen. To help Lara Jean feel less weird about their relationship, Peter suggests they create a second dating contract similar to the one they had when they were fake-dating. The two most important promises in the new contract are that Lara Jean and Peter will always tell each other the truth and will not break each other’s hearts.
The day before winter vacation ends, Lara Jean gets a text from her best friend, Chris, telling her to check an anonymous local Instagram account called Anonyb*tch. The account is famous for posting pictures of people drinking, kissing or misbehaving at parties. Lara Jean discovers a posted video of her and Peter kissing in the hot tub on the ski trip, and she feels humiliated.
Peter gets the video taken down by telling the Anonybtch account that he and Lara Jean are both underage and that keeping a video of them kissing is illegal. Lara Jean suspects it was Gen who took the video and sent it to Anonybtch in the first place. Gen claims that she didn’t take the video, but can’t resist telling Lara Jean that Peter still loves her more than he will ever care for Lara Jean.
Margot reminds Lara Jean that she should do some significant volunteering over the upcoming summer in order to make her college applications look good. Lara Jean wants to volunteer at the Belleview nursing home and contacts the home’s activities director to set up her work schedule. She is excited about her new job, but her mood sours when she discovers that a still shot from the hot tub video is the background wallpaper of all the computers in her school’s computer lab.
The romantic encounter with Petre has become a local internet meme. Just when Lara Jean thinks things can’t get worse, a clip from the video appears in a PowerPoint presentation in front of the entire junior class. Peter rushes onto the stage, grabs a microphone and announces to everyone that he and Lara Jean were not having sex in the video. He adds that he intends to find and punish whoever keeps spreading that video.
Once the rumor mill calms down, Lara Jean really begins to enjoy her work at Belleview. Stormy, one of the residents, wants Lara Jean to meet her great-grandson. Accordingly to Stormy, even though Lara Jean has a boyfriend, she should still keep her options open.
Meanwhile, other matchmaking schemes are being hatched in the Covey household. Kitty turns 10 and invites the Coveys’ neighbor, Ms. Rothschild, over. Ms. Rothschild is a young divorcee, and Mr. Covey is a widower, so Kitty hopes that if she engineers enough chances for her dad and Ms. Rothschild to interact, they’ll fall in love. Lara Jean supports the idea.
On Valentine’s Day, Lara Jean prepares a special valentine card and baked treats for Peter, but when she arrives at school, she discovers him holding Gen by the shoulders and talking with her in a serious tone. Gen looks sad, and Lara Jean isn’t sure whether Peter is just being a good friend to her or whether Gen is trying to win him back. Peter manages to end Lara Jean’s bad mood when he gives her an expensive heart locket from his mom’s antique store, a piece of jewelry that she’s wanted for a long time.
Lara Jean receives an unexpected letter in the mail from her childhood friend John Ambrose McClaren. Months ago Kitty mailed out Lara Jean’s secret love letters to all the boys she’d ever had crushes on. The letters were part of the reason Lara Jean and Peter began fake-dating, and Lara Jean’s middle school crush, John, only recently received his letter.
John wants to reconnect with Lara Jean, so the two start up a written correspondence. Lara Jean becomes nostalgic for the idyllic middle school days when she, Peter, John, Gen and Chris spent all their free time together. Lara Jean recalls that they all buried a time capsule together in seventh grade. She invites all the old crew of friends to dig it up.
When John comes to the time capsule unearthing party, Lara Jean discovers that he’s still as charming and intelligent as he was in middle school. Someone suggests that they all play the game Assassins as a tribute to old times. To play Assassins, they draw each other’s names and secretly try to tag each other out over the course of several days.
According to the rules, the winner will be granted a wish by one of the losers — anything they want, within reason. Lara Jean doesn’t want a wish, but she does want to win the game and finally beat Gen at something. The game becomes complicated when Lara Jean can’t hang out with Peter because he might have drawn her name as his target. Peter and Gen begin hanging out together as part of an alliance to win the game, creating further unease for Lara Jean.
Lara Jean debates the best strategy for tagging out her target, John, since he lives in another city and is harder to contact. To her surprise, he shows up at the Belleview nursing home as Lara Jean is hosting a cocktail hour for the residents. He was invited by Stormy, and it turns out that he is the great-grandson Stormy has been trying to introduce to Lara Jean.
Lara Jean tags John out and recruits his help to get to Peter, her next target. As they hang out at Peter’s lacrosse game, John confesses that he also liked Lara Jean in eighth grade and even made a failed attempt to ask her to the formal dance. Peter doesn’t show up to his own game, and when John and Lara Jean drive to Peter’s house to see if he’s sick, they see Peter hugging Gen, who is sobbing.
Lara Jean doesn’t know why Gen is sad, but she’s angry that Peter is still going out of his way to comfort his ex-girlfriend. Peter later tells Lara Jean that it was Gen who took the video of them in the hot tub, which makes her angry enough to say that she wants to break up. She tags Peter out, which leaves her and Gen as the only two active participants left in Assassins. Peter asks Lara Jean to return the heart locket he bought for her, which she does.
The breakup devastates Lara Jean, and the only thing that gets her mind off her sadness is planning an old-fashioned USO party for Belleview. Chris mentions that Lara Jean shouldn’t pine over losing Peter when she could date John, with whom she has obvious chemistry. John states openly that he likes Lara Jean, but she’s too heartbroken over Peter to consider a new relationship. After a sudden snowstorm strands them overnight at Belleview, John and Lara Jean sneak out onto the front lawn of the nursing home to play in the snow, which Lara Jean finds both fun and romantic.
The night of the USO party arrives. Lara Jean dresses up in 1940s fashion to match the party’s theme. To her surprise, John picks her up wearing a World War II-style uniform. She is impressed by the effort John puts into making the evening special both for her and for the Belleview residents. She is even more excited when he dances with her at the USO party, although he normally hates dancing.
Lara Jean walks into the parking lot after the party and finds Gen and Peter waiting. Gen has tricked Peter into driving her to Belleview so she can win the Assassins game. Before Gen can tag Lara Jean, John pulls up in his car. Lara Jean jumps in and escapes. On the drive home, John kisses Lara Jean, which makes her feel as if she’s falling in love with him.
Lara Jean bakes a cake for a cake walk at Kitty’s school fair. Peter comes to the cake walk with his little brother and asks Lara Jean about her relationship with John. Peter isn’t even sure why Lara Jean broke up with him and wonders if she did it because she was already starting a relationship with John. He accuses her of wanting to have a steady boyfriend as well as a guy on the side.
After the cake walk, Lara Jean sees Gen’s car and follows her in hopes of tagging her out. Gen drives to her father’s office. As Lara Jean watches from the parking lot, Gen’s dad emerges from the building and passionately kisses a young woman who is not his wife. Lara Jean is horrified to recognize that the young woman is a former classmate of her older sister, Margot. She is just 18 years old.
After the couple leaves, Gen confronts Lara Jean in the parking lot. She is angry that Lara Jean saw her father with a girl less than half his age. Gen grabs Lara Jean’s hands and tags her own self out, telling Lara Jean that she’s won the game. Lara Jean realizes that this highly inappropriate affair is the reason Gen has been so brokenhearted recently and also the reason that Peter was holding Gen as she sobbed on the day of the lacrosse game.
On Lara Jean’s 17th birthday, John comes to her school to surprise her. He gives her a beautiful snow globe to commemorate their romantic night playing in the snow in front of Belleview. As Lara Jean thanks John and hugs him, Peter walks up and announces that he also has a present for Lara Jean.
He gives her back the heart locket she returned to him and says that he plans to win her back. Lara Jean says she can’t get back together with him. She leaves with John. She contemplates her relationship with the two boys and finds so many things to adore about each one. She ultimately tells John that Peter won her heart first and that she’s still in love with him.
Fueled by this realization, Lara Jean invites Peter to their childhood treehouse hangout on the day before it's scheduled to be bulldozed. She tells him that she knows about Gen’s secret, which allows Peter to confirm that he was only comforting Gen because she was crying, not because he was dating her behind Lara Jean’s back.
Lara Jean realizes that she has been too obsessed with comparing herself to Gen to really invest in her relationship with Peter. She had been too afraid to really commit to him in case he decided she wasn’t as good as Gen. Lara Jean says that she wants to get back together and Peter agrees, telling her that she’s the only girl he loves. They agree to date wholeheartedly, with no contract, half-measures or attempts at protecting themselves from possible heartbreak.