Starr Carter is a 16-year-old black girl, who lives in the fictional and mostly poor black neighborhood of Garden Heights, but attends an affluent and predominantly white private school, Williamson Prep. After a shooting breaks up a party Starr is attending, she is driven home by her childhood best friend and sometimes crush Khalil. They are stopped by a white police officer, "One-Fifteen". One-Fifteen instructs Khalil, who is black, to exit the car; while outside the car, Khalil leans into the driver-side window to check in on Starr. One-Fifteen assumes he is grabbing a gun and shoots Khalil three times, killing him.
Starr agrees to an interview with police about the shooting after being encouraged by her Uncle Carlos, who is also a detective. Carlos was a father figure to Starr when her father, Maverick, spent three years in prison for gang activity. Following his release, Maverick left the gang and became the owner of the Garden Heights grocery store where Starr and her older half-brother Seven work. Maverick was only allowed to leave his gang, the King Lords, because he confessed to a crime to protect gang-leader King. Widely feared in the neighborhood, King now lives with Seven's mother, Seven's half-sister Kenya, who is friends with Starr, and Kenya's little sister, Lyric.
Khalil's death becomes a national news story. The media portrays Khalil as a gang banger and drug dealer, while portraying One-Fifteen more favorably. Starr's identity as the witness is initially kept secret from everyone outside Starr's family, including her younger brother Sekani. Keeping the secret from her white boyfriend Chris and her best friends Hailey Grant and Maya Yang – who all attend Williamson Prep – weighs on Starr, as does her need to keep her Williamson and Garden Heights personalities separate. Starr's struggles with her identity are further complicated after her mother gets a higher-paying job and the family moves out of Garden Heights.
After a grand jury fails to indict One-Fifteen, Garden Heights erupts into both peaceful protests and riots. The failure of the criminal justice system to hold One-Fifteen accountable pushes Starr to take an increasingly public role, first giving a television interview and then speaking out during the protests, which are met by police in riot gear. Her increasing identification with the people of Garden Heights causes tension with Starr's friends, especially with her boyfriend Chris as well as Hailey, who makes racist comments at Starr. But by the end of the novel, Starr and Maya have started standing up to Hailey's comments while Chris offers support to Starr.
The climax of the novel occurs during the riot following the grand jury decision. Starr, Chris, Seven, and DeVante – a teenager whom Maverick helped leave the King Lords – defend Maverick's store from King, though it does get burned down. The neighborhood stands up to King and as a result of testimony by DeVante, King is arrested and expected to be imprisoned for a lengthy sentence. Starr promises to keep Khalil's memory alive and to continue her advocacy against injustice as Garden Heights rebuilds.
