Fourteen-year-old Arnold Spirit Jr., or “Junior,” lives on the Spokane Indian reservation in Wellpinit, Washington, with his parents, Agnes and Arnold Sr.; his sister, Mary; and his grandmother. The novel is structured as illustrated diary entries by Junior, through which he narrates his history and his current life. Junior was born with hydrocephalus, a condition in which there is excess spinal fluid on the brain. Though it was predicted Junior would die from the condition, he survived the interventionist surgery. However, as a consequence of hydrocephalus, Junior suffered from seizures as a child. Junior has what he considers a large head, a lisp and a stutter, forty-two teeth, and near-sightedness in one eye and farsightedness in the other. Because of these issues, Junior considers himself odd. He is frequently teased by the other children on the reservation and called names such as “hydrohead.”

Junior loves to draw cartoons. He is also very poor, though he doesn’t blame his parents for the family’s poverty. He knows that as dispossessed Spokane Indians, the family has not received the same opportunities as white people. Junior is so poor his family cannot afford to get his sick pet dog, Oscar, treated. Instead, Junior’s father shoots Oscar to put him out of his misery. Junior’s best friend is Rowdy, a boy who often gets into fights. Junior overlooks Rowdy’s violent tendencies because of their friendship and Rowdy’s troubled family. Rowdy’s father beats him and his mother viciously. Additionally, Rowdy is supportive of Junior’s art, which makes Junior love him all the more.

Junior and Rowdy begin their first year at Wellpinit High, the school on the reservation. Junior is especially excited about his geometry class, since he loves to study lines and angles. During the class, Junior discovers that the textbook handed to him by their white teacher, Mr. P, was once signed by Agnes, his mother. He realizes he is so poor he is studying from a thirty-year-old textbook. In anger, he throws the textbook, accidentally hitting Mr. P in the nose. Junior is suspended from school. Mr. P visits Junior’s house and surprisingly apologizes to Junior for the violence. According to Mr. P, Junior became enraged because of the systemic racism he faces and to which even Mr. P has contributed. Mr. P. senses potential in Junior and advises him to leave the reservation school for better academic opportunities. Junior learns from Mr. P that his smart and talented sister, Mary, had once harbored dreams of being a romance novelist. But now Mary seems depressed and has isolated herself in the family’s basement. Junior cannot end up the same way.

Inspired by Mr. P’s words, Junior decides to go to school in Reardan, a white-dominated town twenty-two miles away from the reservation. Junior’s parents agree to the suggestion. When Junior tells Rowdy he is leaving the reservation school immediately, Rowdy is enraged and hits Junior in anger. Junior fears he has lost his best friend.

Junior begins attending Reardan High. He has trouble traveling between home and school, since his father often runs out of gas money. Junior sometimes walks his way or hitchhikes. He develops huge blisters on his feet. At Reardan, Junior faces racist attitudes and comments. He feels out of place in the white-dominated school. However, after he punches the school’s toughest boy in retaliation for an offensive remark, Junior earns the respect of his peers. He develops a crush on his classmate Penelope and reaches out to Gordy, a smart boy in his class. Junior and Gordy become friends.

As Junior settles into Reardan, he begins to feel inadequate on the reservation. He thinks most people consider him a “traitor” for going to school at Reardan. Rowdy continues to be angry with him. On Halloween, Junior goes trick-or-treating on the reservation to raise money for the homeless. He is attacked by three masked boys who beat him and take his money. Junior fears one of the boys may be Rowdy. However, though Rowdy continues to be angry with Junior, he accepts a cartoon Junior brings him.

Close to Thanksgiving, Mary, Junior’s older sister, suddenly leaves home without notice. The family learns that Mary has married a Montana poker player she met at the Spokane casino and is moving to the reservation in Montana with him. Junior’s parents are upset that Mary has left, but Junior thinks Mary is being brave, inspired by his own decision to leave the reservation. Junior begins to receive letters from Mary, in which she describes her life in Montana. Mary is unable to find a job, but she is still happy. At school, Penelope and Junior begin to date. Junior finds increased acceptance at Reardan but hides his poverty out of shame. On a particular outing with his friends, Junior is unable to pay for his and Penelope’s food. He is forced to reveal that he is poor to Roger and Penelope. Much to Junior’s surprise, they sympathize with him. Junior realizes that “if you let people in your life a little, they can be pretty damn amazing.”

Junior earns a place on the Reardan basketball team. Soon, he has to play a game against Wellpinit on the reservation. As Junior and his team enter the gym, the audience turn their back on Junior in an “awesome display of contempt” toward him. Someone in the crowd throws a quarter at Junior, cutting his forehead. Rowdy, on the Wellpinit team, smashes against Junior so hard he gives him a concussion. Reardan loses to Wellpinit. However, in the next match between the teams, Junior’s team wins. Wellpinit drops out of the season, while Reardan makes it to the state playoffs. Junior exults at the underdog Reardan Indians defeating Wellpinit, until he realizes that it is really Wellpinit who are the disadvantaged lot. All of his white teammates have access to iPods and cell phones, unlike the Wellpinit players, who live in great poverty.

Meanwhile, tragedy strikes Junior’s family. His beloved grandmother is killed by a drunk driver on her way home from a powwow. Even as she dies, Junior’s grandmother asks her family to forgive the man who hit her. Eugene is shot in the face by a friend over an argument about the last sip of alcohol. But the worst news the family receives is that Mary and her husband have died in a fire in their trailer. Because they were passed out drunk at the time of the fire, they couldn’t save themselves. Junior’s mother is broken by Mary’s death. Rowdy blames Junior for the tragedy, since he believes Junior’s choice to leave the reservation school prompted Mary’s decision to move out. Junior sinks into grief.

After Mary’s death, Junior receives emotional support from his classmates and teachers at Reardan. Though he feels every planet in his solar system has exploded, he begins to gradually heal. He promises his mother that he will not drink. He manages to earn good grades in his freshman year. Junior also begins to reconnect with his heritage, noting that “the reservation is beautiful.” As the book ends, Rowdy unexpectedly visits Junior. The two friends discuss the past and future and play a game of one-on-one basketball late into the evening.