The film starts with Elle narrating her time with Noah before he leaves for Harvard. Her senior year starts with her classmates gossiping how she and Noah will eventually break up, making her fear elevate as Noah has befriended an attractive girl named Chloe on Instagram. Elle and Lee are planning the kissing booth again for the Charity Fair, while Lee is trying to convince Elle to have Marco Peña, the attractive new transfer student, also known as the new “Noah Flynn” be one of the kissers. Elle beats Marco on the dancing game, forcing him to participate in the booth. Noah suggests Elle apply to Harvard, which clashes with her plans to study at University of California, Berkeley with Lee (where their moms met and became friends). Elle does it without telling Lee.
Elle visits Noah in Boston, meeting his new friends and Chloe, making her more insecure. She finds an earring below Noah’s bed, making her angry and confronting Noah, who assures her nothing has happened between him and Chloe and asking her to trust him. Elle talks with her father about college tuition and learns money is a problem, and is willing to participate with Lee on a dance competition with a cash reward for first place. Lee has an accident on one of their practice sessions (later revealed to be fake) and proposes that Marco become her dance partner, which she isn’t too happy to accept but eventually does. As Marco and Elle start passing time together they grow closer, and an attraction between them initiates.
Unbeknownst to Elle, Lee’s relationship with Rachel is having problems, as Elle is going everywhere with them and not letting them have their time as a couple. After leaving Rachel waiting at the movies she asks Lee to talk with Elle, promising he will but never does. The Halloween Dance arrives, and Lee forgets to tell Rachel that they changed costumes (Rachel was a marshmallow while Lee and Elle were going to be crackers making a smore) upsetting her even more. Elle shares a dance with Marco, almost kissing him but stops when listening to the OMG girls gossiping about her.
The day of the competition, Lee finds in his car trunk Elle’s applications to Harvard, angering him. Elle and Marco excel at their performance and she kisses him at the end of their dance, not knowing Noah was in the crowd, causing him to walk away. Elle wants to go after him the moment she and Marco are announced the winners. Thanksgiving dinner happens at the Flynn’s residence, where Noah arrives with Chloe, upsetting Elle, who is confronted by Lee for not telling him about her application to Harvard and Rachel being upset with her as well. During dinner, Rachel discovers Lee never spoke with Elle and leaves. Lee catches up with her and Rachel breaks up with him. Elle tries to convince Rachel to reconcile with Lee but is unsuccessful. She also gives back the earring to Chloe, who confirms it belongs to her. She reveals to Noah in a bar that one night she slept in Noah’s room when he was out and lost it.
The day of the carnival arrives, and both Lee and Rachel reconcile after being blindfolded and kissing each other at the kissing booth. Elle is also blindfolded and is approached by Marco, who wants to talk with her about their feelings. Elle admits to Marco there is an attraction between them, but tells him she loves Noah and goes looking for him at the airport. Chloe tells her that he went to look for her, and Elle finds him in the park where they first kissed. Noah confesses to being embarrassed that he wasn’t doing as well at Harvard as he initially thought. He also wants a relationship with Chloe in the same way Elle has with Lee. Elle and Noah reunite.
6-7 months later, Noah returns and Elle, Lee, and Rachel graduate. Lee shares with Elle that he was accepted at Berkeley and asks if she received a response. She answers she was waitlisted at both Berkeley and Harvard. As Marco looks from afar at Elle, his friend notices and says she’s not worth it, but Marco decides that she is. When Elle opens both envelopes in her room it turns out she was accepted to both universities, which will force her to make a decision: go to Harvard with Noah or Berkeley with Lee.