The story begins as an unnamed narrator, an algebra teacher, reads of his brother’s arrest for selling heroin. He is deeply disturbed. Thinking of his brother reminds him of his students, who face limited possibilities in a hostile world. At the end of the day he is met at the gate of the school by an old friend of his brother, Sonny; the man is a fellow addict. As they talk, the narrator feels guilty for the dismissive way in which he treats Sonny’s friend. At the end of their conversation, he gives him five dollars.
The narrator fails to write or visit his brother for months. After his daughter's tragic death, the narrator finally writes to Sonny and the two develop a correspondence. When he is released, Sonny returns to Harlem to stay with the narrator.
The narrator reminisces about the brothers' past. He recalls the promise he made to his mother to look after Sonny. He had kept this promise in mind when he returned for her funeral. Despite Sonny’s pleas, he forced his brother to move in with his then-fiancée, Isabel, and her family. Sonny eventually ran away to join the military after being confronted by Isabel’s mother for missing school.
The narrator details his daughter’s death of polio: she collapsed one afternoon, unable to breathe. The narrator’s suffering reminds him of his brother’s trials and allows him to begin to understand what Sonny endured.
Back in the present, the narrator contemplates searching Sonny’s room, presumably for drug paraphernalia, but is stopped by a street revival occurring outside his window. One woman has a particularly moving voice that seems to offer people a brief reprieve from their suffering. Sonny, having been moved by the same scene, invites the narrator to come hear him play music and the narrator accepts. Sonny then explains the universal nature of suffering and the ways drugs and music have helped him cope.
Sonny and the narrator go to the nightclub where Sonny is scheduled to play. Everyone at the club knows and respects Sonny well. Sonny struggles during the first set, but finds his stride in the second set. Sonny plays movingly, making the narrator understand his brother’s suffering. The narrator is reminded of his own suffering, and of his heritage. By understanding Sonny, he has come to understand himself.
