With no money to go drinking at "The Shack," a local youth hangout, on a Saturday night, teenager Twig Webster and his friends, Larry Bronsen, Don Hartlow, Sharon Lee and Mumps Thornberg, decide to crash a party being held by their schoolmate, Stan Osgood, at his parents' home. Though Twig and his gang are not welcomed by their upper class host, Stan is physically intimidated by the intruders and balks at asking them to leave. Soon, Twig's attentions are directed at the beautiful Barbara Nickerson, much to the consternation of her boyfriend, Josh Bickford. When the rowdy party crashers begin to tear up the Osgood home, Josh throws a punch at Twig, and Barbara tells Stan to call the police in order to stop the ensuing melee. Afterward, Josh walks the irate Barbara home, and she admits that she was attracted to Twig's "animal" nature. Back at his own home, Josh is greeted by his mother, whose concern for her emotionally reserved son is not shared by the boy's father, who feels they should respect their son's privacy. Meanwhile, Twig returns to his home just in time to witness his adulterous mother Hazel beat and humiliate his alcoholic father. The next morning, Josh is driven to the Nickersons' home by his father, who advises his son to buy the sports car he has been saving for, though the teenager is unsure of his future, not knowing if he will be drafted into military service or attend college after his high school graduation. Arriving at the Nickersons', Josh is surprised to see Twig, who has been asked over by Barbara's father Jim. After the two boys are forced to make amends to each other for their fight the previous night, Twig invites Josh and Barbara to go with him to "The Shack," though Josh wishes to play golf with their friends at the country club, as they regularly do each Sunday. Barbara, however, insists that they try something different and go to the pub. On his way to "The Shack," Twig stops at home for gas money, only to be told that his drunken father intends to sell his prized sports car if he and his friends crash another party. In direct defiance of his father, Twig convinces all the teenagers at "The Shack" to crash a party being held that night at a nearby lodge. Though Barbara insists on going to the party, Josh tells her he does not think they should. Upset that his credit has been cut off at Clancy's gas station, Twig takes out his aggression on Stan, claiming that he is beating Stan because he called the police the night before. At the lodge, Twig, Barbara, Josh and the other teens gain entrance into the party, but are surprised to discover it is being held by individuals their parents' age. Soon, the teenagers become captives of their intoxicated elders, and when one man tries to force Barbara to dance with him, a fight breaks out between Josh and the drunk. Meanwhile, Twig searches for a way to escape the lodge, only to discover his mother in a motel room with a strange man. Though he begs her to sneak out of the lodge with him, Hazel refuses, and in the subsequent struggle between mother and son, she falls down a flight of stairs and is critically injured. While trying to avoid the arriving police, Twig beats Barbara, demanding that she give him the keys to Josh's mother's car. Hearing her cries, Josh rushes to Barbara's aid, but the panic-stricken Twig beats him until the police appear. Later, at the police station, the Bickfords pledge to be more attentive parents to Josh. After his release from custody, Josh insists on visiting the hospitalized Barbara, and the two teens apologize to each other for the errors of their ways. Informed that his wife has died, Mr. Webster tells the authorities that his son, recently apprehended by the police, was at the lodge at his request. Twig and his father head back to the police station to file a report on the accident, and are reunited by both grief and a new commitment to each other.