Sixteen-year-old Elle and her best friend, Lee Flynn, have been inseparable since birth, born on the same day and raised practically as twins. They serve on their high school's student council and must create a booth for the upcoming Spring Carnival. While brainstorming at Lee's house, they settle on a kissing booth and begin recruiting cheerleaders and jocks to work shifts. Lee's older brother, Noah Flynn, is a senior, star football player, and straight-A student. Widely considered the most attractive boy in school, Noah is known to everyone outside his family simply as "Flynn." Elle once had a crush on Noah but considers it long past, believing he sees her only as his kid brother's friend.
At a party at the Flynn house, several girls pressure Elle to convince Noah to participate in the kissing booth, but he refuses. Elle drinks too much, dances on a pool table, and nearly strips to go skinny-dipping before Noah carries her upstairs to his room and stays with her while she is sick. She wakes the next morning in his bed wearing one of his shirts, with no memory of how she changed. When they accidentally tumble off the bed together, they share an intense moment of eye contact before he pulls away, stirring something in Elle she tries to dismiss.
At school, boys make crude comments about Elle's near-stripping, and when one smacks her backside and another taunts her, Noah intercepts Elle's attempt to punch the boy and punches him instead, sparking a brawl. While waiting outside the vice principal's office, Noah tells Elle he sees her as "like my little sister," extinguishing a small hope she had been nurturing. Elle then learns that Noah has for years warned boys to stay away from her, a fact everyone knew except her. Lee confirms this, explaining that both he and Noah consider Elle too trusting.
When Cody Kennedy becomes the first classmate brave enough to ask Elle on a date, Noah is furious, insisting Cody only wants to take advantage of her. Elle accuses Noah of ruining her love life. The date proves awkward, and Elle chickens out of her first kiss, pecking Cody's cheek instead. They agree to stay friends.
At the Spring Carnival, the kissing booth is a massive success. When Noah unexpectedly appears in line, the girl working the booth panics, and Lee shoves Elle onto the stool as a replacement. Elle and Noah share her first real kiss, and both lose themselves in it as the crowd erupts in cheers. Word spreads instantly, and Elle is bombarded with questions about whether she and Flynn are together.
After the carnival, Elle impulsively kisses Noah at his house, then pulls back in embarrassment. He kisses her back, and they make out intensely until Elle stops things, insisting she will not be just another girl he discards. Noah reveals that his player reputation is largely fabricated: Girls he has kissed at parties claimed they slept with him, but none wanted a genuine relationship with someone known for fighting. He tells Elle she is "all I can think about." They agree to keep their relationship secret from Lee to protect Elle's friendship with him.
Over the following weeks, Elle and Noah meet secretly whenever Lee is absent or occupied with his new girlfriend, Rachel. At a house party, a drunk senior tries to kiss Elle, and Noah punches him hard enough to send the boy to the hospital. Noah takes Elle back to his empty house, where, after telling her he will wait if she is not ready, they sleep together for the first time. The next morning, Elle panics about Lee finding out and resolves to keep the secret while vowing not to fall in love with Noah.
The guilt weighs on Elle. She grows skilled at lying, fabricating excuses for unexplained absences and mysterious texts. She wishes she could confide in her mother, who died in a car accident when Elle was young. Meanwhile, girls at school theorize that Flynn has a mystery girlfriend, and Elle deflects their suspicion.
The secret unravels when Lee walks into a study room and catches Elle and Noah kissing. He accuses Elle of lying for months and choosing his brother over her best friend. When Elle confirms she and Noah slept together, Lee punches Noah and storms out.
That evening, Noah packs a bag and disappears after fighting with Lee at home. Elle confesses everything to her father, who is disappointed but supportive. At midnight, she calls Lee, who tells her he does not hate her but does not like her much right now. He agrees to pick her up for school the next morning, a gesture that brings her to tears of relief.
Noah resurfaces at school days later, exhausted from sleepless nights in a motel. He corners Elle and begs her to take him back, but Elle, afraid of losing Lee again, firmly refuses. She flees, afraid not of Noah but of her own feelings.
On the night of the school's Summer Dance, a masquerade at the upscale Royale hotel, Noah arrives at Elle's door in a tux with a green tie matching her dress. He gets down on one knee, presents a corsage, and asks her to be his date. Elle wavers but accepts. At the dance, she pulls Lee aside to confess the truth. Lee is hurt but offers grudging acceptance, telling Elle that if any girl could change Noah, it would be her, and that he will be there to pick up the pieces no matter what happens. Noah takes the stage, removes his mask, dedicates a song to Elle, and asks into the microphone if she will be his girlfriend. She says yes to cheers from the entire school.
Noah has booked a hotel suite for the night. He tells Elle they do not have to do anything, that he got the room simply to be alone with her. Moved by his sincerity, Elle chooses to stay. The next morning, Lee teases Elle and shares that he confessed his love to Rachel at the dance. Their friendship feels restored.
As summer approaches, Elle begins to recognize the depth of her feelings for Noah but resists admitting them. When Noah calls and says they need to talk, Elle panics and confesses to Lee that she has fallen in love with Noah. Lee reveals he has known for some time. Noah tells Elle he has been accepted to Harvard University for computer science but is torn between going across the country and staying closer to her. Elle insists he must go. Noah takes her on his motorcycle to a hilltop overlooking the city, where they watch the sunset. He tells her he loves her, and she says it back.
Noah graduates, and Elle and Lee celebrate their seventeenth birthday with a costume party. Elle's father gives her a silver watch that belonged to her late mother. The summer passes with Elle and Noah openly together as a couple. At the airport, Noah says goodbye to his family and walks with Elle to the gate. He tells her he loves her, and she replies that she loves him too. Elle acknowledges that their long-distance relationship may not survive but is at peace with uncertainty, choosing to live in the present. Noah disappears through the gate with a final smirk. Lee stands beside Elle, arm around her shoulder, and remarks that everything started "just from the kissing booth." Elle pushes him playfully, and they walk away together, best friends as always.