In the New York City neighborhood of Hellesbay, Billy Kane, Danny Lambeaux, Doug, and Auggie of the Feebas Avenue gang toss a football in the street and smoke marijuana cigarettes. Breaking up the game, fellow members Lon Recchia and Vis run to tell their friends that Lon threw a large cinderblock into an uptown gang’s car. Suddenly, Billy’s neighbor, Mrs. Kahn, crosses the street carrying a paper bag of live crabs. She informs Billy that a new boy, Rex Voorhas Ormine, has moved into the neighborhood, but he may be simple-minded. Just then, the crabs fall onto the asphalt and the boys herd them into a knocked over trashcan. However, Mrs. Kahn no longer wants them and walks away. Billy’s sister, Kate Kane, and Mrs. Kahn’s daughter, Amy, join the boys as Vis decides to kill the crabs by crushing them with a brick, then urinating on them. Meanwhile, newcomer Rex observes the gang from a nearby rooftop and composes a poem aloud, likening them to mutant angels of the streets. The next day at Osmo High School, Billy and Doug talk to classmates Ian and Larry, also known as “Dirt,” about the Marathon Boulevard gang. When English teacher Frank Ellsworth reminds the class of their poetry-writing assignment, the students reply that they did not write anything. In response Frank asks the class to compose poetry spontaneously, saying whatever is on their mind. Ordered to go first, Billy stands up, but cannot think of anything. As he sits down, Dr. Bart Waxman enters the classroom and introduces Rex. As Rex begins to recite a poem, the students laugh. After school, Doug and Billy join Kate, Lon, Vis, and Auggie as they sit outside, discussing how to get tickets to see the Iron Skulls, their favorite hardcore punk rock band. Rex walks by laughing and eating a snack cake. When Vis begins to taunt the boy, Kate tells Vis to leave him alone, but Rex throws his snack cake at Vis’s back. Retaliating, Vis grabs the boy, throws him on the ground, and kicks him. As Vis and Lon run away, Doug and Auggie leave to buy Iron Skull tickets. While Billy and Kate walk off, Rex speaks in verse about a magical world of princesses and beggars. Later that night, an unknown gang breaks into the school and ransacks the classrooms. The next morning, at the start of Frank’s class, school administrator Dr. Morningstone asks the students to reveal what they know about the vandals. Frank then announces that Dr. Waxman will teach a lesson that day. When Rex makes lewd comments and provokes laughter from the class, Waxman becomes frustrated, but Frank intervenes, stating that the lesson is over. That night, Rex walks around chanting the words “The Beat” and imagining the subway under the ground as a beast. Just then, Larry “Dirt” runs up to Rex, asking if he knows where Lon is, and they take off running. Elsewhere, Vis and Danny fight in a boxing match, surrounded by Billy, Lon, Auggie, Doug, and Kate. As “Dirt” arrives with Rex, he tells Lon that the uptown gang shot his friend, “Chessepie.” Lon takes Auggie’s gun, hears a flock of birds flapping, and starts shooting. The group splits up to run away and Billy, Kate, and Rex end up near a fountain in the park. Rex points toward glowing clouds in the sky, and Billy muses that it looks like the world was ruined by atomic war. Rex poetically speculates that the world has been destroyed and the three friends are the sole survivors. The next day at school, Waxman and Frank discuss Rex with Morningstone. While Frank believes he is inspiring the class, Waxman says the boy needs therapy, but Morningstone decides against treatment for the time being. Later that day, Billy asks Rex if he wants to hang out at the Point, a rocky outcrop on the beach. Kate joins them, and notices a small shark swimming near the rocks. Rex believes the shark is there to die and states that Kate, as a priestess, should recite a prayer. Confused, Kate asks Billy what Rex means, but Rex tells her to say the prayer of, “Lemi Leckmi Salma.” Rex recalls being born at the rocks, swimming, walking toward the ruined city, and, gesturing to Billy and Kate, Rex explains that he met a beggar and a princess priestess at the city. When Kate asks who Rex is, he states he is “Voorhas,” then describes the power of music and “The Beat.” Absorbed in his story, the two identify themselves as the beggar and the princess priestess, and refer to Rex as “Voorhas.” The next day in class, Rex narrates a verse about rats, pigeons, babies, and squirrels, prompting a round of applause. Meanwhile, Waxman speaks about Rex with Morningstone, who finally agrees to allow Waxman to begin therapy sessions. That night after the Iron Skulls concert, Rex suggests to Billy, Kate, and Doug that they should perform at the school talent show as “Voorhas,” the beggar and princess priestess, and Doug, as the drummer, to spread the message of “The Beat.” They agree and call their group the Mutants of Sounds. Auggie, Lon, Vis, and Danny then join them, followed by Ian and “Dirt”. Billy suggests a unification of the Marathon Boulevard and Feebas Avenue gangs, and the two gangs agree. Later, Billy and Kate invite Rex over for a snack, and Kate asks for an embrace from “Voorhas.” As he hugs her, she kisses him, and then falls asleep. After, Rex tells Billy that everything around him is vanishing. He has always been locked in dungeons, and scrutinized by people like Waxman. When Billy asks if Rex is speaking about mental institutions, Rex states they are dungeons. Although he knows a new place to escape to, he wants to stay with Billy and Kate. Days later, Frank asks his students if they have poems to read aloud. Everyone raises a hand. Billy reads his poem about the beggar. Impressed, Frank asks for copies of all the poems. That afternoon, Frank is dismayed to learn that Rex will be forsaking his English class for a session with Dr. Waxman, but Morningstone tells him not to fight the decision. The following day, Frank tells Rex to go see Waxman and when the boy refuses, Frank promises he will be back in class tomorrow. At Waxman’s office, Rex throws a tantrum and is sent back to class. The next morning, Kate barges into Frank’s class and tells Rex that men from the River State Hospital Psychiatric Division have arrived for him. Billy and Kate lead Rex away as the class holds off Waxman and the hospital personnel. Kate leaves Billy and Rex hiding in the park. That evening at the school talent show, Waxman informs the students that the Mutants of Sound will not be performing. A riot breaks out, and Kate runs into the rain to find Billy and Rex. She and Billy ask Rex to help, but he states it is over for “Voorhas.” Frustrated, Billy and Kate leave. Meanwhile, at the show, Frank sees the riot and the storm shuts down the power. Suddenly, the crowd stops fighting as Billy appears on stage with a candle, reciting a poem about the mutants of the streets. As Waxman tries to stop him, Frank holds him back. Kate and Doug join Billy, reciting more verses. When they blow out their candles, the lights come on, and the audience begins to chant the words “The Beat.” After the show, the group looks for Rex. By morning they arrive at the Point, and Kate finds Rex's jacket on the rocks. Billy comments that Rex has escaped to his safe new place. They walk away happy, knowing that Rex Voorhas Ormine is free.