High school junior Lara Jean Covey (Lana Condor) finds herself navigating the complex emotions of young love. She expresses her feelings through heartfelt letters that she keeps hidden in her closet, including one addressed to her childhood friend Josh (Israel Broussard), who is currently dating her older sister Margot (Janel Parrish). When Margot breaks up with Josh before heading off to college, Lara Jean struggles with feelings of loneliness and confusion, especially after accidentally revealing her affections through her letters.
One fateful night, while spending time with her mischievous little sister Kitty (Anna Cathcart), Lara Jean falls asleep on the couch. Kitty seizes the opportunity to rummage through Lara Jean's room and stumbles upon the secret stash of letters. The very next day, Lara Jean's life takes an unexpected turn when Peter (Noah Centineo), a former crush, confronts her at school, holding his letter in hand. Overwhelmed by embarrassment, Lara Jean faints. Upon regaining her composure, she is surprised to see Josh approaching her with a letter too. In a moment of panic, she kisses Peter impulsively and runs away, spiraling her into a whirlwind of emotional turmoil.
As the days pass, Lara Jean learns that all of her letters have been sent out, including one to Lucas (Trezzo Mahoro), who reveals he is gay. To navigate her complicated feelings, Lara Jean explains to Peter that she was just attempting to push Josh away. Peter proposes they embark on a fake relationship to make his ex-girlfriend Gen (Emilija Baranac) jealous, to which Lara Jean reluctantly agrees. What begins as a simple façade soon ensnares both of them in a web of genuine emotions, as their classmates, friends, and families buy into their ruse.
However, Peter finds himself genuinely affected when Gen expresses jealousy, and Lara Jean cannot help but feel envious of their interactions. Their close companions, including Lara Jean's father and Christine, point out that this new relationship is bringing out a side of her they have never seen before – one filled with happiness. Eventually, they embark on a school ski trip, where they finally discuss their feelings, leading to an intimate moment in a hot tub. Yet when the trip ends, Gen confronts Lara Jean, revealing that Peter spent the night in her room after their rendezvous.
This revelation devastates Lara Jean, prompting her to end the relationship and return home, only to find that Margot is back from college. A chaotic confrontation ensues when Peter arrives to clarify the situation, just as Josh walks in. Margot overhears the conversation and realizes the depth of Lara Jean's past feelings for Josh, adding another layer of tension to the mix. Just when things can't seem to worsen, a video of Lara Jean and Peter has been posted to Instagram, causing her further distress.
In her moment of need, Lara Jean leans on Margot for support. Kitty eventually admits to sending the letters, which infuriates Lara Jean. However, Margot helps her see that perhaps she subconsciously wanted to send them and reassures her. The sisters reconcile, and together they email Instagram to remove the embarrassing video.
After the Christmas break, Lara Jean realizes that the entire school is aware of the viral video, while Peter tries desperately to clarify that nothing truly happened between him and Gen. Lara Jean bravely confronts Gen, who reveals her motivations were rooted in betrayal over Lara Jean's past kiss during spin-the-bottle. This clash with Gen prompts Lara Jean to reassess her relationships and reclaim her friendship with Josh, paving the way for her to confront her own feelings for Peter. Eventually, with a gentle nudge from Kitty, Lara Jean musters the courage to see Peter. He admits his love for her, culminating in a passionate kiss as they step forward together.
In a light-hearted twist during the mid-credits scene, John (Jordan Burtchett), another letter recipient who Lara Jean knew from fifth grade, comes to her door bearing flowers, hinting at the excitement and unpredictability of young love continuing to bloom.
