Brad Hamilton (Judge Reinhold) is a popular senior who is looking forward to his last year of school and almost has his 1960 Buick LeSabre paid off. He has a part-time job at All-American Burger, a fast food joint where his girlfriend, Lisa, also works. This esteemed establishment has a strict policy of etiquette: "No Shirt, No Shoes, No Dice." Brad is subsequently fired for losing his temper at an obnoxious customer. When Brad tries to tell Lisa how much he needs her, she says she is breaking up with him to see other guys. Brad quits his job at Captain Hook Fish & Chips because of the humiliation of having to wear a pirate costume when delivering food. He later gets a job at Mi-T-Mart, where he successfully thwarts an attempted robbery and is subsequently promoted to store manager.
Brad's sister Stacy (Jennifer Jason Leigh) is a 15-year-old sophomore and also a virgin. Stacy works at a pizza parlor at the mall alongside her outspoken friend, popular and sexually active Linda (Phoebe Cates). 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 nineteen. She then sneaks out to meet him and they have sex in a dugout at a softball field. Stacy never hears from Ron again, revealing the loss of her virginity to Linda.
Mike Damone (Robert Romanus), a dilettante who earns money taking bets and scalping tickets, fancies himself a sagacious and worldly ladies' man. After Mike's friend Mark Ratner (Brian Backer) proclaims his love for Stacy to him, Mike lets him in on his five secrets for picking up girls. Mike later coaxes Mark into taking Stacy on a date to a German restaurant. Afterwards, at her home, Stacy invites Mark into her bedroom, where they look at Stacy's photo album together. They eventually kiss, but he soon shies away and leaves after Stacy attempts to initiate intimacy with him. Stacy invites Damone over after school for a swim in the pool, which leads to sex in the pool house between the two. Stacy later informs Damone that she is pregnant as a result. But on the day of her appointment, embarrassed at being unable to raise the money for his half of the bill, Damone ignores her. Stacy asks Brad to drive her supposedly to a bowling alley, but she goes to the abortion clinic. When Brad returns, Stacy makes him promise not to tell their parents. When Stacy tells Linda, Linda becomes angry at Damone, leading to an almost physical confrontation between Damone and Mark in the boys' locker room until it is broken up by the vice principal.
Jeff Spicoli (Sean Penn) is a surfer and habitual marijuana user who runs afoul of a strict history teacher Mr. Hand (Ray Walston), who is intolerant of Spicoli's disregard for his class. One night, Spicoli wrecks Ridgemont star football player Charles Jefferson's 1979 Chevrolet Camaro Z28 during a joyride with Jefferson's younger brother. Spicoli decides to park the car in front of the school with slurs painted on it supposedly written by Ridgemont's rival, Lincoln High. When Ridgemont plays Lincoln, Jefferson is angry about his car, thrashes several of Lincoln's players, and wins the game for Ridgemont. 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 session that lasts until Mr. Hand is satisfied that Spicoli has understood the lesson.
In addition to Mark and Stacy's continuing relationship and Brad's promotion to manager at the convenience store, fates of some of the other characters are revealed in an epilogue sequence prior to the closing credits: Spicoli saves Brooke Shields from drowning and then spends the reward money hiring Van Halen to play at his birthday party. Linda gets accepted to UC Riverside and moves in with her Abnormal Psychology professor. Damone gets arrested for scalping Ozzy Osbourne tickets and gets a job at 7-Eleven when he gets out, and Mr. Hand still believes everyone is on drugs.

Question: Who is Stacy in relation to Brad?
Answer: Brad's sister