Brad Hamilton is a popular senior at Ridgemont High School and looks forward to his final year of school. He has a job at All-American Burger, almost has his 1960 Buick LeSabre paid off, and plans to break up with his girlfriend Lisa so he can be completely eligible during the year. Brad, however, is fired for threatening an obnoxious customer. While he tries to tell Lisa how much he needs her, she informs Brad that she wants to break up with him to date other guys. Brad gets a job at Captain Hook Fish & Chips, but quits in humiliation when a beautiful older woman laughs at his wearing a pirate costume while making a food delivery.
Brad’s sister Stacy is a 15-year-old sophomore and a virgin. She works at a pizza parlor at Ridgemont Mall alongside her outspoken friend, Linda Barrett. One night at work, Stacy takes an order from Ron Johnson, a 26-year-old stereo salesman, who asks her out after she tells him she’s 19. She later sneaks out of her house to meet him and she loses her virginity to him in a dugout at a baseball field. She tells Linda about the date, stating how much it hurt. Linda offers advice to Stacy on the matter, which she often does as the more worldly and experienced of the two. Ron sends her flowers the next day, but stops calling after November.
Mike Damone, a smooth-talking know-it-all who earns money taking sports bets and scalping concert tickets, who fancies himself as a sagacious and worldly ladies’ man. His shy but amiable best friend, Mark Ratner, works as an usher at the movie theater across from the pizza parlor at the mall. When Mark proclaims his love for Stacy to Mike, Mike lets Mark in on his five secrets for picking up girls. Mike later persuades Mark to ask Stacy out on a date to a German restaurant. Afterwards, at her home, Stacy invites Mark into her bedroom, where they look at her photo album together. They begin to kiss, but Mark abruptly leaves after Stacy attempts to seduce him. She mistakenly interprets his shyness as lack of interest. Stacy tells Linda that she doesn’t believe Mark is interested yet she likes him. She is advised to find someone else to like fast. After he shows her a small act of kindness, Stacy grows interested in Damone. Eventually she invites him to go swimming in her pool, which leads to them having sex in the pool house during which he ejaculates very quickly. Brad, who has become sullen and withdrawn since his breakup, is caught by Linda masturbating in the bathroom after he daydreams about her after seeing her in a bikini.
Stacy later informs Damone that she is pregnant, and he tells her she wanted sex more than he did, which she denies. She asks for him to pay half the money for an abortion and give her ride to the clinic, to which he agrees. However, unable to come up with his half despite attempts to call in debts owed from his business dealings, he bails on her the day of the appointment. She asks Brad to drive her to a bowling alley to meet friends, but he sees her cross the street to the abortion clinic. Brad waits for Stacy and he confronts her about the abortion. Stacy makes Brad promise not to tell their parents, but doesn’t divulge who got her pregnant. When Stacy tells Linda that Damone bailed on her and didn’t pay his half, Linda becomes furious. The next day, Damone finds both his car and locker vandalized as revenge. Mark confronts Damone about his involvement with Stacy. They almost get into a fight, but their gym teacher breaks it up.
Jeff Spicoli is a carefree stoner and surfer who runs afoul of strict history teacher Mr. Hand, who is intolerant of Spicoli’s disregard of his classroom rules. One night during a joyride with his friend, Spicoli wrecks the 1979 Chevrolet Camaro Z28 of Ridgemont star football player Charles Jefferson. Spicoli covers up the damage by making it look like the car was trashed by fans of Ridgemont’s sports rival, Lincoln High. When Ridgemont plays Lincoln, Jefferson (furious about his car) thrashes several of Lincoln’s players and almost single-handedly wins the game. On the evening of the graduation dance, Mr. Hand shows up at Spicoli’s house and informs him that since he has wasted eight hours of class time over the past year, Mr. Hand intends to make up for it that night. They have a one-on-one history session that lasts until Mr. Hand is satisfied that Spicoli has understood the lesson.
In the end, Mark and Stacy start dating, and Mark and Damone make peace. Brad takes a job at a convenience store and is promoted to manager after foiling a robbery with the unintended help of Spicoli.