Awkward high school sophomore Samantha "Sam" Baker (Molly Ringwald) struggles to get through the day of her 16th birthday, which her entire family forgets because her older sister, Ginny (Blanche Baker), is getting married the next day. Sam is also plagued by her ongoing infatuation with popular and attractive senior Jake Ryan (Michael Schoeffling). Her day at school fares no better when she finds out that her completed "sex quiz," which she surreptitiously slipped to her friend, never reached her (and, unbeknownst to either of them, was intercepted by Jake Ryan himself). Sam panics, as the quiz contains personal information, including the fact that she is a virgin and is saving herself for Jake. At the same time, she doesn't know that Jake has the hots for her as well, but is too insecure to even talk to her.
Samantha has a whole new set of problems when she arrives home to find that both sets of grandparents are staying for the duration of the wedding visit. On top of it all, one set of grandparents brings along bizarre Chinese exchange student Long Duc Dong (Gedde Watanabe). Sam's grandparents force her to take him along to her school dance that night and, to Sam's amazement, it takes "The Donger" only five minutes to find an unlikely girlfriend: the tallish, large-breasted jock, Marlene, promptly nicknamed "Lumberjack."
A running subplot involves a geeky freshman, (Anthony Michael Hall) who continually (and unsuccessfully) tries to bed his love interest, Sam, to satisfy a bet with his friends. The character is referred to on several occasions in the movie as either "Ted" or "Farmer Ted", but he is credited solely as "the Geek."
In the auto-shop room during the dance, Sam and Ted begin talking, and Sam confesses her love for Jake. Upon hearing this, Ted tells her that Jake has been asking about her at the dance, and they agree that Sam should just go and talk to him. As she's leaving, Ted reveals the wager to Sam, who, in her excited state, agrees to loan him her panties to help him win a dozen floppy disks.
Later (after a peepshow of Sam's panties for $1 admission, which she does not find out about until the next day), Ted and his equally unwelcome friends, Cliff (Darren Harris) and Bryce (John Cusack), crash the senior after-party at Jake's house during which the entire house is completely trashed. A small group of beer-chugging jocks pin Farmer Ted under a glass table, abduct his two geek friends, and take them for a joyride in the trunk of their car before releasing them unharmed.
At night's end, Jake finds Ted trapped under the table and they begin to talk. Jake inquires further about Sam, and Ted explains the situation. Jake makes a deal with Ted: If Ted lets Jake keep Sam's panties, then he will let Ted drive home his inebriated, stuck-up, prom queen girlfriend, Carolyn Mulford (Haviland Morris), in Jake's father's Rolls Royce. They never get home because the very drunk Carolyn makes moves on Ted while he's driving before passing out. Ted the Geek takes the opportunity to drive over to Cliff and Bryce's house to ask them to take a photo of him with Carolyn together in the back seat of the car.
At Samantha's house the next morning, after some ensuing madness with all her obnoxious relatives, the family eventually makes up before the wedding and apologizes for forgetting her birthday. They drive to the church, where a very nervous Ginny takes a few too many tranquilizers and causes a scene throughout the wedding ceremony.
Meanwhile, Jake goes on a search for Samantha at her house, only to find Long Duck Dong there getting over a bad hangover, who tells him where Sam went. Jake finds Carolyn and Ted the Geek passed out in the back seat of the Rolls in a nearby parking lot, where he uses the excuse of finding them together to break up with Carolyn (who had surprisingly fallen for Ted, and thus doesn't mind the breakup very much).
Afterward, Jake drives to the church just in time to meet an incredulous Sam after her sister's wedding. Here, Jake and Samantha finally meet face to face for the first time, and Jake invites her over to his house rather than to Ginny's wedding reception. Samantha accepts. The movie concludes with Jake and Samantha sharing a kiss over a birthday cake with 16 candles.