The story centers around tough, young street thug, Rusty-James, and his idol, The Motorcycle Boy (who we eventually find out is his older brother). Rusty-James finds out that a rival gang member, Biff, wants to fight him, so they meet in a vacant lot. Although Biff has a knife, Rusty-James initially wins the fight; then, The Motorcycle Boy returns after having been away in California, warning all of the gang members present that his ban on rumbling is still on. While Rusty-James is distracted, Biff cuts him with his knife; The Motorcycle Boy then breaks Biff's arm.
The two brothers return home, where The Motorcycle Boy tends to Rusty-James' injuries. Rusty-James is expelled from school; the brothers, along with Rusty-James' friend, Steve, get drunk across the river, where The Motorcycle Boy saves the other two from being mugged. But, the two brothers are growing apart once again, and Steve tells Rusty-James that he can no longer be a part of his gang-related activities. Rusty-James also learns that another gang member, Smokey, is taking over the leadership of the gang.
Searching for The Motorcycle Boy, Rusty-James finds him in the local pet store, staring at Siamese fighting fish--"rumble fish," the older brother calls them. Later that night, The Motorcycle Boy breaks into the store and frees all of the animals. He attempts to free the rumble fish and release them in the river, but he is shot dead by police. Rusty-James is arrested and sent to a reformatory. The story ends in California--where it opens in Chapter 1--with Rusty-James and Steve reuniting, but only briefly.