In Baltimore, 1959, a tight-knit group—Modell, Eddie, Shrevie, Boogie, and Fenwick—head to a Christmas dance before driving to their usual late-night haunt, Fell's Point Diner. On the way, Fenwick stages a pretend car crash, which annoys the others. Boogie is a hairdresser and law student; he has placed a $2,000 bet on a basketball game and refuses Bagel's offer to call it off. Modell accepts a ride home from Eddie, while the others pick up Billy, who has come to town to stand as Eddie's best man for a New Year's Eve wedding.
The next morning, Billy reunites with Eddie at his mother's house, and they visit their old pool hall. [Shrevie] discusses married life with Eddie at the diner; Eddie is preparing a test of football knowledge for his fiancée Elyse to determine if they will marry. Having lost his basketball bet, Boogie wagers with his friends that he can have sex with Carol. The group parts ways in the morning, and Boogie and Fenwick encounter an equestrienne named Jane. Meeting Barbara at church, Billy learns she is pregnant, the result of an unexpected night they spent together; she tries to dissuade him from feeling obligated to marry her.
Watching College Bowl, Fenwick is surprisingly knowledgeable and offers to help pay Boogie's debt. Shrevie loses his temper at Beth for disturbing his record collection and storms out. Boogie consoles her, and Fenwick goes to ask his brother for money, but he refuses. Shrevie pulls Eddie and Billy from a screening of The Seventh Seal to corral Fenwick, who has drunkenly occupied the church's Nativity scene in his underwear, leading to their arrest.
In custody, Eddie reveals that Elyse is taking the test the following night, and Billy faces down a belligerent drunk. Eddie, Shrevie, and Billy are bailed out by their fathers, but Fenwick's leaves him overnight. They meet Boogie at the diner, and he deduces Eddie is still a virgin, while Billy and Barbara discuss their predicament at the TV station. A lonely Beth visits Boogie at the hair salon, and he is threatened by his bookie, Tank. Learning Carol has the flu, which jeopardizes his bet, Boogie reminisces with Beth about their own past relationship, and they make plans for a tryst while everyone is busy with Elyse's test.
Shrevie, Fenwick, Billy, and Modell, along with Eddie's and Elyse's parents, await the results of the football test: Elyse fails by two points, and Eddie calls off the wedding. Boogie brings Beth a wig, secretly planning to disguise her as Carol to win the bet, and drives her to Fenwick's apartment. Fenwick and Shrevie hide in the closet to verify the encounter, but Boogie tells Beth the truth, urging her to work things out with Shrevie. Eddie and Billy visit a strip club, and Boogie arrives at the diner, where Shrevie and Fenwick warn him Tank is waiting. Boogie is ready to accept the consequences, but Tank reveals that Bagel has paid off the debt.
Billy commandeers a piano to lead the entire strip club in a rousing number, to Eddie's delight. There, Eddie decides to marry Elyse after all, and Billy assures Barbara he genuinely loves her. The wedding proceeds, themed around Eddie's beloved Baltimore Colts: Shrevie and Beth reconcile; Boogie, Billy, and Fenwick bring Jane, Barbara, and Diane; and Modell delivers a heartfelt speech. The movie ends as Elyse—whose face is never seen throughout the movie—tosses the bouquet, which lands on the friends' table.
