The Absolutely True Diary of A Part Time Indian is a young adult novel told from the perspective of Native American teenager Arnold Spirit Jr., known as Junior, who is an aspiring cartoonist. Though born and raised on the Spokane Indian Reservation, Junior begins attending an all-white public school on the recommendation of one of his teachers. The novel chronicles Junior's attempts to not only fit in at his new school but also face traditional challenges of adolescence in this poignant and darkly humorous story.
The book follows one school year in the life of Junior, a fourteen-year-old boy living with his family on the Spokane Indian Reservation near Wellpinit, Washington; it is told in episodic diary style, moving from the start of the school year through the major holidays and through to the beginning of summer. It includes both Junior's written record of his life and drawings he makes, some of them comically commenting on his situations, and others more seriously depicting important people in his life.
The diary begins by setting up Junior's circumstances, including the fact that Junior was born with hydrocephalus and therefore is small for his age and suffers from seizures, poor eyesight, stuttering, and lisping, and therefore has always been picked on by other people on the reservation. His family is poor, a condition Junior attributes to being from the reservation and not having opportunities to fulfill their potential; their poverty is symbolized early when Junior's dog Oscar gets heat stroke and has to be put down by his father because they cannot afford to take him to a veterinarian. The only child friend he has is Rowdy, a classmate who is abused at home but stands up for Junior and also lets Junior see his vulnerable side, such as his enjoyment of such kids' comics as Archie and Richie Rich.
The diary then moves to Junior's first day of high school and to the incident that sets up the plot of the book: when his geometry teacher, Mr. P, hands out the textbooks, Junior sees his mother's maiden name written in his, meaning that the textbook is at least thirty years old. Angered and saddened by the fact that the reservation is so poor that it cannot afford new textbooks, Junior violently throws the book, which hits Mr. P's face, breaking his nose. In the ensuing family visit, Mr. P convinces Junior to transfer to Reardan High School, as the school in the reservation is very poor. Reardon High School is in the countryside, with children that come from wealthier families. Junior is the only Indian at Reardan besides the team mascot. Although Junior's family is poor, and although the school is 22 miles away and transportation is unreliable, they support him and do what they can to make it possible for him to stay in the new school. Rowdy, however, is upset by Junior's decision to transfer, and the best friends have very little contact during the year.
Junior develops a crush on the school's most popular white girl, Penelope, and becomes study friends with a smart student named Gordy. His interactions with the white students give him a better perspective both on white culture and his own. He realizes how much stronger his family ties are than those of his white classmates, noticing that many of the white fathers never come to their children's school events. He also realizes that the white students have different rules than those he grew up with, such as when he reacts to an insult from the school's star athlete, Roger, by punching him in the face. Junior hit him because that's what he had been taught he was expected to do, and he expects Roger to get revenge. But Roger never does; in fact, Roger and his friends show Junior more respect. Junior also gets closer to Penelope, which makes him more popular with the other girls at the school.
Roger suggests that Junior try out for the basketball team, and to Junior's surprise, he makes the varsity team, which puts him against his former school, Wellpinit, and specifically Rowdy, who is Wellpinit's star freshman. Their first match demonstrates to Junior just how angry the reservation people are at him for transferring: when he enters the court, they turn their backs on him. During the game, Rowdy elbows Junior in the head and knocks him unconscious. In their second match that year, Reardan wins and Junior gets to block Rowdy. Junior feels triumphant until he sees the Wellpinit players' faces after their defeat and remembers the difficulties they face at home and their lack of hope for a future; ashamed, he runs to the locker room, where he vomits and then breaks down in tears.
In the course of the year, Junior and his family suffer many tragedies, all related to alcohol abuse. His grandmother is hit and killed by a drunk driver, a fellow Indian; family friend Eugene is shot in the face by his friend Bobby after fighting over alcohol; and his sister and her husband die when their mobile home is accidentally set on fire after a night of heavy drinking. These events test Junior's sense of hope for a better future and make him wonder about the darker aspects of reservation culture. But they also help him see how much his family and his new friends love him, and he learns to see himself as both Indian and American. Meanwhile, Rowdy has realized that Junior is the only nomad on the reservation, which makes him more of a "traditional" Indian than everyone else in town. In the end, Junior and Rowdy reconcile while playing basketball and resolve to correspond no matter where the future takes them.