While out on an early jog near a racetrack, Charley Thompson, a 15-year-old who has recently relocated with his troubled single father, Ray, to Portland, Oregon, finds himself taking on a casual job caring for a racehorse named Lean On Pete. The horse's owner, Del, is a grumpy individual who assigns Charley all the hard labor at the stables, though he tries to connect by gifting Charley a pair of boots and teaching him about table manners. As they bond, Charley confides in him about not knowing his birth mother and how Ray forbids him from seeing his Aunt Margy, his only maternal figure.
Tragedy strikes one evening when a confrontational Samoan man breaks into their home, attacking Ray for his affair with the man’s wife. Ray suffers severe injuries, getting pushed through a window, and is left with dangerous glass lacerations. After a hospital visit reveals Ray's grave septic infection, he urges Charley to take a job with Del, despite Charley’s insistence on staying by his side.
In a twist of fate, Bonnie, Del’s top jockey, rides Pete to victory, yet it becomes increasingly apparent that with Pete’s age, he will soon be sold for slaughter in Mexico. After returning home, Charley is heartbroken to learn of Ray's death. Left with Ray’s belt, Charley flees the hospital before he can be taken by Social Services. Soon he discovers that Pete has lost his race and is about to be sold. In desperation, Charley confronts Del and Bonnie about buying Pete, but they dismiss his pleas. Driven by a strong desire to protect the horse, Charley takes matters into his own hands by stealing Del's truck and Pete before embarking on a journey to Wyoming in search of Aunt Margy.
After a few days on the road, Charley quickly depletes his funds and resorts to siphoning gas and dining-and-dashing to stay afloat. When the truck breaks down, he is left with no choice but to continue on foot through an unforgiving desert with Pete by his side. Their travels lead them to the doorstep of Mike and Dallas, two Army veterans who offer them food and shelter. However, feeling uneasy about their hospitality, Charley sneaks out in the dead of night. The next morning, calamity strikes as Pete is startled by some motorcycles and gets hit by a car on a desert road, leading to his tragic death. Grieving deeply, Charley attempts to evade the police who arrive at the scene to assist him.
Finding his way to a new town, Charley breaks into a house in search of water and a chance to launder his clothes. As a homeless youth, he finds temporary refuge with a struggling couple, Silver and Martha, but things quickly take a turn when Silver attacks Charley over his saved money. In a fit of rage, Charley retrieves his cash after a violent encounter and hops a bus to Laramie, Wyoming, where he ultimately reunites with his Aunt Margy.
That night, haunted by his tumultuous journey, Charley confides in his aunt about the crimes he committed along the way and reveals his nightmares concerning the untimely deaths of both Pete and Ray. In a heartfelt moment, Aunt Margy comforts him as he lets his emotions flow, crying for his losses. As time passes, Charley finds solace in his new routine, going for morning runs and finally appreciating the tranquility of his new surroundings.
