Born on the same day in the same hospital, Elle Evans (Joey King) and Lee Flynn (Joel Courtney) have been best friends all their lives. To protect their special relationship, the pair created a set of friendship rules they are sworn to follow including #9: relatives totally off limits, especially Lee’s bad boy older brother Noah (Jacob Elordi). Soon a romance with Noah blossoms and Elle finds herself breaking the rules and hurting Lee. Quickly, tensions flare between the two brothers and Elle faces losing them both.
On the first day of school Elle’s pants rip forcing her to go to school wearing a skirt that is too small. Another student named Tuppen touches her which almost gets Lee into a fight but Noah intervenes before Lee could get hurt. Elle is dress coded and has to serve in detention with Noah and Tuppen where the latter apologizes. She later agrees to go out with him however she is stood up. He goes to see her later that night to let her know that he was threatened by Noah not to go out with her.
Elle and Lee have to create a booth for the school carnival and decide on a kissing booth. The two have difficulty finding people to participate but get a group of popular girls called the OMG’s (Olivia, Mia, and Gwyneth) to participate by lying to them saying that Noah likes one of them and will go if they go.
That night, the kissing booth is a success. However, girls including the OMG’s become angry when they discover that Noah is not actually working the kissing booth. For payback, the girls send Elle up to the booth to kiss a nerdy kid while blindfolded. The boy switches with Noah having him next in line to kiss Elle. When Elle won’t stop talking Noah kisses her, not knowing who it is she takes off her blindfold and Noah goes in for a second kiss. After the kiss Elle goes to tell Lee what happened and he is upset but knows it’s for charity. He tells Elle he is fine with it as long as she doesn’t end up rubbing “choochies” with him. If she does he said he would never talk to her again. After cleaning up Elle sees Noah flirting with another girl so she runs home while Noah looks at her. While on her way home it starts to rain and Noah offers her a ride on his motorcycle. However, the rain gets to heavy and they have to stop. Elle kisses Noah again and he pulls away. After staring at each other Noah goes in for another kiss. Elle ends up pulling away, telling him she can’t be another one of his conquests. He looks hurt but tells Elle why he likes her. They kiss again but are interrupted by a security guard who recognises Noah and tells him to stop bringing girls there. Elle gets upset and leaves.