Rusty-James, or Rusty, runs into his old friend, Steve Hays, at the beach. Five or six years have passed since they last saw each other. Steve is in college and Rusty is not long out of the reformatory. Rusty’s memory is not very good these days. When Steve looks at the scar on Rusty’s side, Rusty tells him that he got it in a knife fight. Steve remembers. He tells Rusty he was there when it happened several years before. When Steve mentions that Rusty looks just like someone from their past, Rusty thinks he could have been happy to see Steve again if he had not made him remember everything.
Rusty tells his story. At the age of fourteen, Rusty is hanging around Benny’s, playing pool with his friends when he learns that Biff Wilcox wants to kill him. Rusty is not afraid of Biff and seems to be annoyed that Biff wants to kill him for the comments he made about a girl named Anita. He tells his friends what he said, and when the gang agrees that Rusty is telling the truth, the notion of fighting about it seems silly. Some of the boys, though, are holding on to the past, the days when gang fights were common. Steve tries to impart a warning to Rusty about gang fights, a warning handed down by Rusty’s older brother, Motorcycle Boy, a former gang leader.
Rusty gets angry with Steve for bringing up Motorcycle Boy and makes plans to fight Biff. Rusty kills a few hours before the fight by spending some time with his girlfriend, Patty. He falls asleep while there, nearly missing the fight. Later, Rusty arrives to fight Biff, and is accompanied by his friends Steve, Smokey Bennet, and B. J. Jackson. Biff, too, brings some friends for backup. Biff’s erratic behavior leads Rusty to believe that he is on drugs, which causes him to worry that the fight will not be a fair one. Rusty’s fears are confirmed when Biff pulls a knife. Rusty is able to knock the knife away from Biff and beats him until it appears the fight is over. Motorcycle Boy arrives and announces his return. Rusty is momentarily distracted and vulnerable to being attacked. Biff seizes the opportunity to grab the knife and stabs Rusty in the side. Motorcycle Boy steps in and ends the fight by breaking Biff’s wrist.
Motorcycle Boy and Steve manage to get Rusty home to the apartment the boys share with their mostly absent alcoholic father. They bandage Rusty’s wounds. Motorcycle Boy talks about his recent trip to California. Rusty falls asleep and dreams about his older brother. Rusty is uncomfortable being himself and is preoccupied with becoming just like his brother.
Despite the knife wound, Rusty shows up for school the next day. After school, he steals a set of hubcaps from a car near Benny’s, while Steve talks about his mother’s recent hospitalization. The owner of the car notices the theft in progress and begins chasing the boys, accompanied by a couple of his friends. Rusty and Steve barely escape. The ordeal scares Steve, and he cries as they walk home. His crying scares Rusty because he has never seen a guy cry before. Rusty assumes that Steve is crying about his mother, but Rusty never knew his own mother, so he cannot relate.
Later that evening, Rusty and Motorcycle Boy chat with their father about the older son’s trip to California; his girlfriend, Cassandra; and one of the local cops, who has issues with the brothers. Rusty goes to a late-night party at the beach. He ditches school the next morning and gets expelled. When Patty finds out that Rusty had been with another girl at the party, she breaks up with him. Rusty feels there is nothing he can do to change what has happened, so he copes by “forgetting” these events. He also recruits Steve and Motorcycle Boy to go out drinking, also to help him forget this terrible day.
Motorcycle Boy tells Rusty that while he was in California, he saw their mother. Rusty has only vague memories of her because she abandoned the family when he was two years old. Rusty’s father had begun drinking when his mother left, even going on a three-day drinking binge and leaving Rusty home alone. Motorcycle Boy surmises that this is why Rusty fears being alone now.
After watching Motorcycle Boy play pool, Rusty and Steve get separated from him and end up in an unfamiliar part of town. The boys are mugged and Rusty is beaten. Once again, Motorcycle Boy saves them. The following day, Rusty notices that his vision is not quite right, most likely because of the head injury he suffered in the mugging. Rusty goes to Steve’s house and discovers that Steve’s father had beaten his friend severely for staying out past curfew the night before. Rusty tells Steve that he is worried about his brother, but is not sure why. He asks Steve to help keep an eye on him. Steve refuses because he cannot afford to do anything that would cause him to get into more trouble at home. Rusty leaves, and he will not see Steve until they meet at the beach in five years.
Rusty goes to Benny’s and finds out that Patty is now dating his friend Smokey. Smokey had set Rusty up by inviting the other girls to get Patty to break up with him. Instead of being angry, Rusty envies Smokey for being smart enough to think up that kind of a plan. B. J. tells Rusty that Motorcycle Boy is in the pet store looking at the fish. Rusty goes to the pet store, and the two watch the fish. Motorcycle Boy calls them rumble fish because they would kill each other if they could. He wonders if the fish would still act that way if they were in the river.
Later that night, Motorcycle Boy breaks into the pet store and starts setting the animals free. Rusty tries to stop him, but it is no use. Motorcycle Boy grabs the rumble fish and heads for the river as police arrive. An officer fires a warning shot that hits him. He dies near the river with the rumble fish flopping on the ground, dying beside him. Rusty knows the shooting was intentional. He screams and smashes his fists through the window of the police car.
Rusty is back on the beach with Steve, five years later. Steve asks him if he ever went back home after his brother’s death. Rusty says no. Steve invites him out for dinner and tells him where to meet later. Rusty decides that he never wants to see Steve again, so that he can start forgetting about Motorcycle Boy, but it is taking Rusty longer than expected to forget his older brother.