In suburban Chicago in 1984, high school sophomore Samantha "Sam" Baker is hopeful her 16th birthday is the beginning of a great new year, but is shocked when her family forgets the occasion because her older, beautiful, self-absorbed sister Ginny is getting married the next day.
At school, Sam fills out a friend’s sex quiz where she reveals her crush on senior Jake Ryan. Meanwhile, Jake, having noticed Sam’s looks at him, asks his friend Rock about her. Rock dismisses her as an immature child, but Jake says he is frustrated by his girlfriend Caroline’s partying ways. On the bus ride home, Sam fends off repeated flirtations from geeky freshman Ted.
At home, Sam's day gets worse when she discovers she must sleep on the sofa because her grandparents and a foreign exchange student named Long Duk Dong are all staying at the house for the wedding. She is further upset when her grandparents also do not remember her birthday and have Dong go with her to a dance at school that night.
At the dance, Sam pines for Jake while Dong has attracted the powerful and strong jock, Marlene. Ted, in an effort to impress his friends Bryce and Wease, dances with Sam, who runs off in tears. In an effort to salvage his reputation with all the geeks, Ted bets Bryce and Wease a dozen floppy disks that he will get physical with Sam before the dance ends. As proof, Bryce and Wease demand Sam’s underwear. Jake asks Ted about Sam, having seen them dancing.
Ted apologizes to Sam who opens up about her family forgetting her birthday and her crush on Jake. Ted tells her that Jake asked about her and Sam is shocked and asks what Ted thinks she should do. Despite his genuine interest in Sam, Ted encourages her to talk to Jake, and she agrees. Before she leaves, he gets her underwear to win his bet and he, Bryce, and Wease charge the other freshmen boys a dollar to see it. Meanwhile, Sam tries to approach Jake, but loses her nerve and runs off. Jake and Caroline leave the dance, leaving Sam thinking Jake doesn’t like her.
At Jake's house, Caroline and her friends have started a wild party. Jake, angry with Caroline, retreats to his bedroom and tries calling Sam, but her grandparents yell at him for waking them up and tell him Sam isn't interested. After the party, Jake is furious at the damage left behind. He finds Ted hiding under a glass coffee table after he knocked over some jocks’ beer can pyramid. Ted tells him Sam is interested in him and Jake confesses that he has lost interest in Caroline. He takes Sam’s underwear from Ted and, in exchange, lets Ted take a drunken Caroline home in his father’s Rolls Royce. To further impress the geeks, Ted stops at Bryce and Wease’s house to get Wease to take a picture of him with Caroline in the expensive car, but the finished picture only reveals the top of Ted’s head.
Sam’s father apologizes for forgetting her birthday and tells her that if Jake doesn’t see what a wonderful person she is, then he isn’t worth Sam’s time. She lies on the couch thinking of Jake not knowing that he is also thinking of her.
The next morning, Sam's mother apologizes to Sam for forgetting her birthday and everyone heads to the church for the wedding. Jake arrives at Sam's house where a hungover Dong miscommunicates that Sam is at church getting married. Jake finds Caroline and Ted making out in the back of his dad's banged-up car and break up. Jake surprises Sam at the church after the wedding and invites her back to his house.
That night, Jake gives Sam her underwear and a birthday cake with 16 candles on it. He tells her to make a wish and she tells him her wish already came true. They kiss as the film fades to black.