The unnamed narrator of the story discovers from a newspaper that his younger brother, Sonny, has been arrested for selling and using heroin. As he prepares to teach his algebra class, the narrator remembers Sonny as a young boy. His students, he realizes, could someday end up like Sonny, given the obstacles and hardships they face growing up in Harlem. At the end of the school day, the narrator heads home, but he notices that one of Sonny’s old friends, who is always high and dirty, is waiting for him by the school. The two men walk together, talking about Sonny. The narrator simultaneously hates and pities Sonny’s friend, who, despite his problems, makes it painfully clear to the narrator just how difficult Sonny’s drug-addicted life has been and how difficult it will continue to be.
Time passes, but the narrator never writes to Sonny in prison until the narrator’s young daughter, Grace, dies. Sonny writes a long letter back to his brother in which he tries to explain how he ended up where he is. The two brothers then stay in constant communication. When Sonny gets out of jail, the narrator is there for him. He takes Sonny back to his own family’s apartment.
In an extended flashback, the narrator recalls how Sonny and their father used to fight with each other because they were so similar in spirit. He remembers the last day he saw his mother while on leave from the army, when she told him to watch out for his brother. She told him that when his father was a young man, he watched his own brother get run down by a car full of white men who never bothered to stop. The experience traumatized and damaged the narrator’s father for the rest of his life.
After that conversation with his mother, the narrator went back into the army and didn’t think about his brother again until their mother died. After the funeral, the two brothers sat and talked about Sonny’s future. Sonny told his brother about his dream of becoming a jazz pianist, which the narrator dismissed. The narrator arranged for Sonny to live with his wife’s family until Sonny graduated from college. Sonny reluctantly agreed to do so. He didn’t want to live in the house and spent all his spare time playing the piano. Although Sonny loved the music, the rest of family had a hard time bearing his constant practicing.
While living with his sister-in-law, Sonny got into trouble for skipping school. He tried to hide the truancy letters, but one eventually made it to the house. When his sister-in-law’s mother confronted him, Sonny admitted to spending all his time in Greenwich Village, hanging out with musicians. The two fought, and Sonny realized what a burden he’d been on the family. After two days, Sonny joined the navy. The narrator didn’t know whether Sonny was dead or alive until he received a postcard from Greece. After the war, the two brothers returned to New York, but they didn’t see each other for quite some time. When they eventually met, they fought about Sonny’s decisions in life. After one especially difficult fight, Sonny told his brother that he could consider him dead from that point on. The narrator walked away, telling himself that one day Sonny would need his help. The flashback ends there.
After having Sonny live with him for a few weeks, the narrator debates whether he should search Sonny’s room. As he paces back and forth, he sees a street-corner revival occurring outside his window and thinks about its significance. Eventually Sonny comes home and invites his brother to watch him perform later that evening. The two brothers go to a small jazz club where everyone knows and respects Sonny. Sonny and the band get on stage and play, and as they play, the narrator watches Sonny struggle with the music. He watches all his brother’s struggles come pouring out as he plays, and only then does he finally realize who Sonny is and what he’s made of.