In Cincinnati, Ohio, Matt Larkin (Andrew McCarthy) is a popular engineering college student engaged to a beautiful and wealthy woman named Alice (Chiara Peacock). One afternoon, Matt's best friend, Tipton (Ben Stiller), convinces him to attend a party at a backwoods country house just across the Kentucky border. Upon their arrival, however, they find only the remnants of the previous evening's gathering and meet the hostess, Jean (Patti D'Arbanville). While getting a beer for Tipton, Matt chats with Jewel (Molly Ringwald), a 20-year-old high school dropout who spends most of her time at the house to avoid her drunken stepfather.
Over the next few weeks, Matt develops an infatuation with Jewel, and they begin a passionate affair. Realizing he is disillusioned with his life, he calls off his engagement to Alice, garnering disapproval from his family and friends.
One evening, Jewel tells Matt she cannot see him because she has to visit her ailing grandmother. Although Matt laughs off the clichéd excuse, Tipton later tells him that Jewel is actually 16-years-old and secretly married. When Matt confronts her, Jewel refutes the rumor about her age, but admits she is married to an abusive older man named Green (Viggo Mortenson). She insists they have not consummated the relationship, however, because she fears being around Green and his ill-tempered father. Remembering that Tipton's uncle is a lawyer, Matt consults with him about a possible annulment.
Despite their differences in class and education, Matt moves into an abandoned train depot near Jean's house, which he and Jewel use as a romantic meeting place. Sometime later, a college acquaintance named Sproles (Doug Hutchinson) tells Matt that Green is a lowlife who is suspected of murder. Tipton insists Jewel is manipulating Matt in order to escape her lower class upbringing. Although reluctant to believe Sproles, Matt moves out of the train depot and returns to school, leaving behind an envelope filled with cash for Jewel.
Hoping to distract himself from thoughts of her, Matt and Tipton spend an evening partying with a group of girls, including a friend named Ellen (Molly Hagan), who has always harbored feelings for Matt. As the hour grows late, Matt suggests they move the gathering to the abandoned train depot, where the girls roast marshmallows and joke about Matt's rumored relationship with a mysterious married woman. Suddenly, Jewel barges in and asks to speak to Matt outside. Demanding to know why he left her, Matt somberly replies, "I didn't. I just tried to." He allows her to borrow his car so she can give Green the annulment papers, and kisses her goodbye. After Jewel leaves, Tipton attempts to convince his friend to pursue Ellen instead, but Matt is unable to shake his attachment to Jewel.
The next morning, Matt learns that Green and a group of his friends, including Sproles, raped Jewel outside a bar after he refused to sign the annulment papers. Although Matt insists they go to police, Jewel wants to forget what happened. Matt confronts Sproles, who claims he only witnessed the incident while hanging out at the bar with his brother, an undercover policeman, and says that Jewel "misinterpreted" what happened. Unsure what to believe, Matt forces Sproles to show him where Green lives.
Inside the house, Matt confronts Green and is surprised to find Jewel lurking in the adjacent room. She hands Green the annulment papers, declaring that she intends to leave him whether or not he signs them. Although she does not claim to be educated, Jewel was always smart enough to know her relationship with Matt was doomed from the start, and walks away to begin her own life.
One year later.
Matt enrolls in graduate school and reconciles with Tipton, who is now expecting a child with his longtime girl friend, Maureen. While out ice skating, he runs into Jewel and her new boyfriend, Stephen (Kent Poole). Revealing that she is working toward completing her high school degree, she kisses Matt on the cheek and whispers, "Thank you." As he watches Jewel and Stephen skate onto the rink, Matt wipes away a tear and disappears into the crowd.