Sean Parker, a 15-year-old ninth grader, lives with his abusive mother, who has an alcohol addiction, and has not seen his father in two years. When his English teacher, Mrs. Walker, leads a class discussion about heroes, Sean describes Chris Saunders, a classmate killed the previous year, as someone real, brave, loyal, and fair. Later that day, Sean hears that Rick, a former friend with gang aspirations, plans to fight him. Sean confronts Rick and provokes a fistfight that security guards break up before Rick can pull a blade.
In the assistant principal's office, Dr. Bushel explains that new legislation allows schools to expel chronically disruptive students. Both boys receive a behavior contract and five-day suspension: three days out of school, two in-house. Mrs. Walker tells Sean she sees confusion, anger, intelligence, and stubbornness in him and assigns a two-page hero essay. Dr. Bushel reveals he has contacted Sean's estranged father. A flashback shows Sean's childhood: After his father left, his mother physically abused him, breaking his pinkie finger and leaving bruises. His father never believed Sean's accounts and kept returning him to his mother. When his father arrives at school, Sean perceives only obligation in his blank eyes and walks out.
That Friday, Sean is caught walking home past curfew after a party, his fourth violation in six months. A judge sentences him to 80 hours of community service at Carbondale Ranch. His mother picks him up, slaps him, and locks him in his room until Monday, barely providing food.
On Monday, a police van delivers Sean to the ranch. Mr. Hassler, the ranch's elderly owner, is a stern, white-haired man with bright blue eyes and a military bearing. He assigns Sean to muck out stalls, telling him shoveling manure is all he is good for right now. Sean grudgingly cleans the barns. Over his first home-cooked meal in days, Mr. Hassler reveals he served in the Army during World War II.
That afternoon, James, Mr. Hassler's ranch hand, announces that a mare named Manda has gone into premature labor. Mr. Hassler forces Sean to help with the breech delivery, in which the foal has not turned to the proper position for birth. After three hours, the foal is born alive, but Manda lunges at it, ripping Mr. Hassler's shirt and cutting his arm. They rush the foal to a separate stall, since Manda has killed or injured previous foals. Left alone with the colt, Sean recognizes his own need for help in the animal's pleading eyes. Mr. Hassler explains that the colt has "imprinted" on Sean, meaning the foal sees Sean as his parent. Sean names the colt Knicker after the nickering sound the foal makes.
Tuesday evening, Sean's mother demands money. When he explains his community service is unpaid, she slaps him and slams him into a wall; the back of his head strikes a rusty nail. He passes out and wakes to find his room ransacked. He runs into the night and jogs to the ranch. Wednesday morning, Mr. Hassler finds Sean asleep in Knicker's stall, the colt's head resting on him. Knicker, who had been listless from grief over losing his mother, appears revived. Sean collapses with a concussion and infection from the nail wound.
Sean wakes in a bed at the ranch house. In the living room, he notices a glass case containing military medals, a Purple Heart, a bullet, and shrapnel. He insists on visiting Knicker and wins the argument. That night, Sean tearfully begs Mr. Hassler not to send Knicker back to Manda, but his pleas transparently reveal his own situation: He is begging not to be returned to his own abusive mother. The next morning, Mr. Hassler quietly announces they will not reintroduce Knicker to Manda.
Over the following days, Sean trains Knicker to accept a halter, a head strap used to lead a horse, and settles into ranch life. Mrs. Walker visits the ranch, where she once worked during high school. Sean overhears her telling Mr. Hassler that not all of Sean's bruises come from peers. That evening, Mr. Hassler shares a war story: In 1943, he encountered a wounded German soldier, and neither man could shoot the other. Mr. Hassler applied a tourniquet; the soldier shared his chocolate. As they parted, the German gave Mr. Hassler his last bullet, the most important item in his display case.
Friday night, Sean's mother is hospitalized with kidney failure from her alcohol addiction. At the hospital, Sean's father appears, offers cab money, and dismisses Mr. Hassler. Sean accuses his father of abandonment and walks out. Mr. Hassler stays and hits Sean's father after the man calls Sean a "worthless case," prompting threats of a lawsuit that could jeopardize the ranch. On the drive home, the two argue fiercely in a frozen field. Mr. Hassler accuses Sean of lacking the courage to win and declares he put his entire ranch on the line for Sean. Sean asks to stay and promises to work hard, thanking Mr. Hassler by comparing his generosity to the German soldier's gift of his most precious possession.
The next morning, Mr. Hassler shares his own past: He grew up during the Depression in a family affected by alcohol addiction and crime, was falsely accused of theft, and enlisted in the army at 16 to escape. He urges Sean not to sink into what others expect. That afternoon, Judge Frank—the judge who sentenced Sean to community service and a longtime friend of Mr. Hassler's who arranged for Sean to be placed at the ranch—visits, and Sean tells the judge his full story. The judge reveals that Sean's father claims he left to build a custody case and was blocked from contact by Sean's mother. Sean will not return to his mother, and living arrangements with his father will be worked out.
Saturday evening, Sean enters Manda's stall while cleaning. She blocks the doorway and bares her teeth. Something inside Sean snaps: He strikes her repeatedly with a shovel, screaming and crying until James pulls him off. Mr. Hassler drives Sean to the hospital for stitches on a bite wound to his shoulder. He does not yell but suggests Sean attacked Manda because of what she did to Knicker and what Sean's mother did to him. A doctor legally required to report suspected child abuse questions Sean about his injuries; Sean tells his story with Mr. Hassler present.
Sean buys a puppy for Mr. Hassler, who names it Cody. He calls his father; both apologize and arrange to meet. On Monday, Sean returns to school, where Rick warns of unfinished business. That evening, Sean finds Rick in the barn with a pistol, a bag of drugs hidden in Knicker's stall. When Mr. Hassler approaches, Rick fires a warning shot and threatens to kill Mr. Hassler and Knicker. The instant the gun swings away from Mr. Hassler, Sean lunges at Rick. The gun fires, and Sean is shot in the chest. Through fading consciousness, he sees Mr. Hassler disarm Rick and feels Knicker's warm breath on his neck.
In a third-person epilogue, Mrs. Walker visits Sean's hospital room. Mr. Hassler sits at his bedside, and Sean's father has been visiting. Sean regained consciousness that morning, and the doctors are hopeful. Mrs. Walker gives Mr. Hassler Sean's hero essay, which argues that heroes no longer exist because society has replaced loyalty and self-sacrifice with greed and self-interest. The essay concludes: "My hero is...somewhere, I hope" (221).