As the story opens, an unnamed narrator reads in a newspaper about the arrest of his brother Sonny for using and selling heroin. This unnerving revelation causes him to think back to their childhood, when Sonny was "wild, but he wasn't crazy" (4).
The narrator is sort of shell-shocked the whole day as he tries to teach his classes. (He's a high school math teacher and he can't help but compare Sonny to the guys in his class.) He somehow makes it through the day and as he's getting ready to leave he runs into one of Sonny's old friends, who seems like he wants to get something off his chest. It turns out that he feels a little responsible for what's happened to Sonny, since he is a heroin user himself. He was honest about how good it feels to get high when Sonny asked him about it. He also tries to explain to the narrator what it's like to be on drugs and why Sonny may have gotten hooked, but the narrator just ends up more frustrated and angry.
It's some time after this that the narrator finally tries to contact Sonny. He only does so because his (the narrator's) young daughter, Grace, has died. Sonny replies right away and tells the narrator that he really, really needed to hear from him but he never got in touch because he figured he had caused too much pain. Sonny returns to Harlem (where the two men grew up) and moves in with the narrator and his family once he gets out of jail.
Things are a little tense and awkward at first, but the narrator's wife, Isabel, is able to break the ice with Sonny. Things are still a bit off, though. After a few weeks, the narrator is home alone and considers searching Sonny's room for signs that he's still doing drugs. But before he gets around to it, he notices a street-side religious revival taking place outside his apartment. He watches two women and a man sing and pray, and then he notices that Sonny has been standing on the street watching. Once the revival finishes, Sonny heads up to the apartment and he and the narrator get into what's probably an inevitable argument.
Sonny starts to talk about suffering and about trying to escape it by using drugs. He talks about playing the piano and how sometimes he just has to play. He tries to explain to his brother why he turned to drugs in the first place, but the narrator doesn't want to hear it at first. He blames the music (and other musicians) for leading Sonny to heroin, and he tells Sonny how angry he is that Sonny seems determined to end his life by being an addict. Sonny gets just as angry – for his brother never reaching out to him after his arrest, for not accepting that people have different ways of dealing with things, and for not understanding that being a musician isn't what turned Sonny into a drug addict. The two eventually cool off and the narrator realizes that he's just worried about his little brother, so he promises himself that he'll always look out for Sonny from here on out. At the end of this conversation, Sonny invites the narrator to come hear him play at a club that night.
When they get to the club, an old musician named Creole greets Sonny and tells him that he's been waiting for his return. Lots of people in the club know Sonny and have come to hear him play after his long absence. The narrator suddenly realizes that he's in Sonny's world now. Creole is the bandleader at the club. He gets the other musicians ready and then leads Sonny onto the stage. This is a big moment for Sonny, and he's nervous. His piano playing is shaky and unsure, but after he gets through the first set he suddenly becomes his old self again, and his playing mesmerizes the narrator. As the narrator sits at a table by himself, he finally gets what Sonny's been trying to tell him all along – about music, about being a musician, about trying to deal with suffering. He sends Sonny a drink (Scotch and milk – gross!) and as the waitress puts it on the piano above Sonny's head, the narrator doesn't think he sees it. But then Sonny takes a drink, nods to the narrator, and goes back to playing.