"Sonny's Blues" is a story written in the first-person singular narrative style. The story opens with the narrator, who reads about his younger brother named Sonny who has been caught in a heroin bust. The narrator then goes about his day; he is a teacher at a school in Harlem. However, he cannot get his mind off Sonny. He thinks about all the boys in his class, who don't have bright futures and are most likely doing drugs, just like Sonny. After school, he meets a friend of Sonny's, who tells him that they will lock him up and make him detox, but eventually he will be let out and be all alone.
Originally, the narrator doesn't write to Sonny. After his daughter Grace dies of polio, he decides to write Sonny a letter. Then Sonny writes back, so they get in contact again. At this point, we learn how Sonny is related to the narrator—they are brothers. They keep in contact, and after Sonny gets out of jail, he goes to live with the narrator and his family. They eat a family dinner, which then turns into a flashback about their parents.
The narrator describes his father, a drunken man, who died when Sonny was fifteen. Sonny and his father had the same privacy; however, they did not get along. Sonny was withdrawn and quiet, while their loud-talking father pretended to be big and tough.
The narrator then thinks back to the last time he saw his mother alive, just before he went off to war (most likely fought in Korea). She told him the story of how his uncle died (he was run over by some drunken white men, possibly deliberately), how his father was never the same, and that the narrator has to watch over Sonny. The narrator was married to Isabel two days after this talk, and then he went off to war. The next time he came back to the states was for his mother's funeral.
When he comes back for the funeral, he has a talk with Sonny, trying to figure out who he is, because they are so distant from one another. He asks Sonny what he wants to do, and Sonny replies that he wants to be a jazz musician and play the piano. The narrator does not understand this dream and does not think it is good enough for Sonny. They also try to figure out his living arrangement for the remainder of his high school career. Both of these subjects lead to an argument. Sonny calls his brother ignorant for not knowing who Charlie Parker is, and argues that he does not want to finish high school or live at Isabel's parents' house. Eventually, however, they find a compromise: Isabel's parents have a piano, which Sonny can play whenever he wants, provided he goes to school. Sonny, begrudgingly (but somewhat excited about the piano), agrees.
Sonny stays at Isabel's and supposedly goes to school. When he gets home, he constantly plays the piano. Sonny, however, is more like a ghost; he shows no emotion and does not talk to anyone.
It is soon found out that Sonny is not going to school. Instead, he is going over to Greenwich Village, and hanging with his jazz friends (and most likely doing drugs). Once Isabel's parents find this out, Sonny leaves their house, drops out of school, and joins the navy.
They both get back from the war and live in New York for a while. They see each other intermittently, and whenever do, they fight. Because of these fights, they do not talk to each other for a very long time.
It then flashes forward, and he talks about Gracie and her polio affliction. It was then that the narrator decided to write to Sonny. It seems that the narrator could better understand his brother now. ("My trouble made his real.")
It then flashes forward to what we would assume is the present. It's a Saturday and Isabel is gone with the children to visit their grandparents. The narrator is contemplating searching Sonny's room and begins to describe a revival meeting that both he and Sonny are watching. There is a woman singing, which seems to hypnotize them both.
Sonny comes into the house, and asks the narrator if he wants to come and watch him play in Greenwich Village, and the narrator, unsure, somewhat begrudgingly agrees to go.
Sonny then begins to talk about his heroin addiction in somewhat ambiguous terms. He says that when the lady was singing at the revival meeting, it reminded him what it feels like when heroin is coursing through your veins. Sonny says it makes you feel in control, and sometimes you just have to feel that way. The narrator asks if he has to feel like that to play. He answers that some people do. They talk about suffering. And the narrator asks Sonny if it is worth killing yourself, just trying to escape suffering. Sonny says he is not going to die faster than anyone else trying not to suffer. Sonny divulges that the reason he wanted to leave Harlem was to escape the drugs.
They go to the jazz club in Greenwich Village. The narrator realizes how revered Sonny is there. He hears Sonny play. In the beginning, he falters, as he has not played for over a year, but after a while, his playing becomes completely magical and enchants 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 connecting moment. His brother finally understands that it is through music that Sonny is able to turn his suffering into something worthwhile.