Preston Meyers and his best friend, Denise, meet up after their graduation ceremony. Preston has just learned that Amanda Beckett, a girl he's been infatuated with for the past four years, is now single. He talks Denise into going to the huge after-graduation bash at Molly's house so he can give Amanda a love letter he's been working on for years. Of course, Molly's parents are not home, so the party is a free-for-all.
Also at the party is Kenny Fisher, a.k.a. "Special K." Kenny is a short, white kid who acts like a black kid. He speaks the lingo and wears all the hip, urban clothes. Kenny used to be Denise's best friend in grade school, but he moved on when they got into middle school. He wanted to be a cool kid, so he left her behind and even harassed her in high school.
Amanda's ex-boyfriend, Mike Dexter, is at the party as well. He decided to break up with Amanda because he'd soon be going off to college and having sex with all kinds of other girls. He tries to get his three best friends, all jock-types, to break up with their girlfriends, too.
Another guy at the party is the school geek and misfit William Lichter. Poor William has been tortured and bullied for years by Mike. He's out for revenge as he and his nerdy friends plan to get Mike drunk and take naked pictures of him and his friends in a gay embrace.
At the house party, Preston tries to get up the nerve to talk to Amanda, he leaves Denise on her own. We find that she hasn't really got any friends and no one at the party knows her. After realizing she doesn't belong, she decides to leave. But a brownie smacks her in the side of the head, so she has to seek a bathroom to clean up.
Meanwhile, Kenny is determined to lose his virginity at the party and he's narrowed it down to 10 "lucky" finalists. Once he realizes that none of them want to get with him, he decides that anyone will do. He hears a girl say that she wanted to get back at her cheating boyfriend and she was going to sleep with the next guy who talked to her. 'Special K' takes advantage of overhearing the conversation and goes to freshen up in the bathroom. Unfortunately, there is a huge line, so he gets Molly to let him upstairs (where NO ONE is supposed to go) to use that bathroom. The doorknob is broken, so he shouldn't shut the door all the way.
Kenny and Denise end up locked in the bathroom and we learn all about their past and get to see them reconcile. They eventually have sex and then get in an argument about it.
Meanwhile, Preston is having a hard time talking to Amanda, who is being consoled by her airhead friends and hit on by every guy in the place. At one point, a friend embarrasses Preston in front of Amanda by talking about how he'd puked in his backpack years before.
Mike meets up with a guy named Trip McNeely, who was a "sexual icon" in his high school years. Trip tells Mike that the biggest mistake he made was breaking up with his long-term girlfriend when he went to college because he thought he'd be getting with all kinds of girls there. Mike realizes he made a mistake with Amanda and tries to get back with her. She refuses him and in his humiliation he starts drinking heavily.
William sees his opportunity, but is getting drunk himself. They spend time talking about things and Mike apologizes to William about being a jerk to him all these years.
Amanda is talking to her cousin Ron about Mike, and Ron grabs her and kiss her. Preston goes inside with the letter just in time to see this scene. Heartbroken, he leaves. Amanda pushes Ron away, disgusted. Outside, a shattered Preston tosses the letter in the trash and heads home.
Amanda sits down inside the house, wondering what is next. Preston's letter is knocked out of the trash and gets stuck to a girl's shoe. After being brought into the house, it ends up in front of Amanda. She reads it and melts. But she has no idea who Preston is. She goes home and begins tearing down the memories she had with Mike. Preston's letter helped her realize she was special and that she was worth much more than just being Mike's girlfriend.
Also part of the story is the band scheduled to play during the party. They are excited that perhaps an acquaintance of a famous rocker may be there. As they begin to play, they have a fight about what some of the band members are wearing. Eventually, they decide to get back together, only as soon as they begin to finally play, the police show up to bust the party.
When the cops show up at the party, it is way out of control. William tells Mike they have to get out of there and they run outside. Unfortunately, Mike runs to the pool house which is where William's nerdy friends are waiting to chloroform Mike and his buddies for the picture-taking. They ambush Mike and William and take gay pictures of them before they realize what they've done. Mike and William end up in jail.
Denise is out walking home as Kenny drives up beside her. He says he's sorry for the fight they had and they make up and head to a diner for breakfast. Outside the diner, Denise goes to talk to Preston about what happened and he tells her Amanda didn't feel the same way.
William wakes up in jail and learns from an officer that Mike has taken all the heat, saying he beat William up and took the naked pictures of him. The next morning, William heads to the diner to thank Mike, but Mike is acting like his old self again, telling William to get lost. Stung by Mike's betrayal, William walks out of the diner, vowing never to see or talk to Mike again.
Preston goes home and the next day he heads to the train station to go off to school. Amanda finds him there and thanks him for the letter. They decide that since he's leaving, they can't be together. She says she should probably be alone for a while anyway. As she walks away, they both turn and run to each other. They decide to try to make it work long-distance. They write to each other every day and have never been happier.