Brother opens with Michael, the novel's narrator and protagonist, recounting the time his older brother, Francis, took him to climb a hydroelectricity pole that gives a great view of their city. Francis warns Michael about the danger of touching two live wires and becoming a conduit for the electricity coursing around them. Francis emphasizes the importance of following him closely and always remembering.
In the present day, Michael lives with his mother, Ruth, in the same apartment they have always occupied in a low-income neighborhood of Scarborough, Ontario, known as the Park. A childhood friend and former girlfriend named Aisha comes home to mourn her father, who died recently of cancer. Although Michael invites Aisha to stay with him and Ruth, he regrets the decision once Aisha is in their home. Michael cannot hide from Aisha that he has dedicated his twenties to caring for his mentally ill mother, who hasn’t come to terms with the loss of her other son, Francis.
In flashbacks, Michael intersperses the present-day narrative of Aisha's visit with episodes from his childhood. Michael and Francis’s parents are Trinidadian immigrants of Black and South Asian descent living in Canada. After their father abandons the family, Michael’s mother gives up trying to qualify as a nurse and has to work many cleaning jobs to pay the bills. When their mother goes to work, the boys explore the neighborhood. When they are young teenagers, Francis stops attending school and begins earning money to contribute to the household. Their mother learns that he hangs out at Desirea's, a barbershop in a strip mall. She slaps him in the face and reprimands him for associating with "criminals."
Around this same time, Francis and Michael are out of the house one night when they hear gunshots. A local boy named Anton rounds the corner and falls hard, hit by another, fatal, shot. The police interview the boys, who didn't see the suspects. The event unnerves Francis. Later that week, he announces he is going away but will send money back to Michael and Ruth.
In the present-day narrative, Michael is aware of how people in their community speak of him and his mother as damaged by what happened to Francis. Michael works at a local supermarket, stocking shelves in a job that isn't lucrative or rewarding. Aisha encourages Michael to get Ruth some help in coming to terms with Francis's death, as both Ruth and Michael refuse to address what happened. Michael resists it under the guise of protecting his fragile mother from destabilizing conversations.
In the past, Francis leaves home to bunk at Desirea's. Michael finds him there and the two go out for a meal that Michael is surprised Francis can afford. Francis brings Michael to the apartment of their father, who Francis has tracked down. When they arrive, however, their father pretends they have the wrong person.
After that night, Michael is welcome to hang out at Desirea's with his brother and mentor. Michael also begins a romantic relationship with Aisha, who he usually hangs out with at the local library, an air-conditioned refuge from that summer's oppressive heat. As part of the crew, Michael is there when Jelly, a DJ, auditions to perform at a big hip-hop concert coming to town. While Jelly's skills are impressive to his friends, the mix of disparate genres doesn't wow the judges as much as Francis hoped it would. Francis gets into a physical fight with the bouncers when they refuse to let him back in to talk with the promoters.
Badly roughed up, Francis refuses to go to the hospital out of fear that he will be punished by the police for fighting. He seeks medical attention at Desirea's. Concerned with Francis's seemingly concussed state, Dru, the barbershop's owner, calls for an ambulance. Instead, the police arrive in tactical gear, having interpreted the call as indicating that people are currently fighting in the barbershop, which they have recently searched for drugs (finding nothing). Dru explains the situation, but Francis defies the police's authority, refusing to act demure to defuse the charged atmosphere.
The police tell everyone to get against the wall to be patted down, but Francis won't obey the orders: he demands to know what he's done wrong to deserve their harsh treatment. Michael and Francis's friends try to get Francis to listen to the cops, but he won't. Michael notices cops un-holstering their guns as Francis acts erratically. When a cop grabs Jelly's arm, his other arm on his gun, Francis shouts at him not to touch Jelly and reaches to stop the hand going for the weapon. Another cop shoots Francis dead in what investigators later deem a lawful killing.
Following Francis's killing, Ruth and Michael isolate themselves from other people and insulate themselves in denial of their grief. Michael stops talking to Aisha, who moves away to pursue university and jobs that have her travel the world. Francis's friends try to stay in touch with Michael and Ruth, but Michael pushes them away until most people leave them alone in their stunned sadness.
As Michael comes to terms with his brother's death, the present-day narrative sees Ruth wander in front of a moving car and get struck down. Michael stays up all night in the emergency ward, relieved to learn that she only has a fractured femur. Although Michael has been rude to them and told them to leave him and Ruth alone, Jelly and Aisha are waiting by the hospital entrance with flowers picked from the Rouge Valley, a stretch of nature that cuts through their neighborhood. The novel ends with the four characters eating together and listening to records in Ruth's sitting room. Aisha enthusiastically plans for them all to visit the Rouge together, where spring has come at last.
