"Sonny's Blues" is a story written in the first-person singular narrative style. Much of the story is told through a series of flashbacks as memory and family history are revealed to be central drivers of the trauma and alienation experienced by Sonny and the Narrator.
The story opens with the unnamed narrator reading about a heroin bust resulting in the arrest of a man named Sonny, his brother. The narrator goes about his day as an algebra teacher at a high school in Harlem, but begins to ponder Sonny's fate and worry about the boys in his class. After school, he meets a friend of Sonny, who laments Sonny will struggle with addiction even after his detox and release.
After the narrator's daughter Grace dies of polio, he finally reaches out to Sonny. The narrator remembers leaving for the war, leaving Sonny with his wife Isabel and her parents. Sonny decides to play the piano, and his passion is obsessive. Once Isabel's parents find out that Sonny has not been attending school, he leaves their house, drops out of school, and joins the Navy.
Sonny returns from the war. Their relationship sours, as the narrator intermittently fights with Sonny.
Back in the present, the narrator reveals that Grace's death has caused him to reflect on his role as an older brother, surmising that his absence impaired Sonny's personal growth. The narrator resolves to reconcile with Sonny.
While Isabel takes her children to see their grandparents, the narrator contemplates searching Sonny's room. He changes his mind, however, when he sees Sonny in a revival meeting in the street below his apartment, where a woman sings with a tambourine alongside her brother and sister, and enraptures the audience.
Some time later, Sonny invites the narrator to watch him play in Greenwich Village. The narrator begrudgingly agrees to go. Sonny explains his heroin addiction in vague analogies. The woman's performance reminded him of the rush he got using heroin, equating it to a need to feel in control. The narrator asks Sonny if he has to feel like that to play. Sonny answers that some people do. The narrator then asks Sonny if it is worth killing himself just to try to escape suffering. Sonny replies that he will not die faster than anyone else trying not to suffer. Sonny reveals that the reason he wanted to leave Harlem was to escape the drugs.
The brothers go to the jazz club in Greenwich Village. The narrator realizes how revered Sonny is there as he hears him play. In the beginning, Sonny falters, as he has not played for over a year, but his playing eventually proves to be brilliant and he wins over the narrator and everyone in the club. The narrator sends a cup of Scotch and milk up to the piano for Sonny and the two share a brief moment of bonding. The narrator finally understands it is through music that Sonny is able to turn his suffering into something worthwhile.
But the story ends with an ominous symbol: the Cup of Trembling, which leaves readers suspecting that the brothers will continue to face challenges in spite of this moment of harmony.
