Plot

While participating in a rehabilitation program training wild mustangs, a convict at first struggles to connect with the horses and his fellow inmates, but he learns to confront his violent past as he soothes an especially feisty horse.

In the inconclusive assessment in his initial placement as reintegration into the general population at the Nevada correctional facility where he's been housed, long term inmate Roman Coleman, a brooding loner, is assigned to outdoor maintenance, which largely entails shoveling the manure from around the animal enclosures. Noticing his seemingly natural draw to the horses, Myles asks that Coleman be reassigned to his rehabilitation program: the training of captured wild mustangs ultimately to be sold at auction. The program matches each inmate solely to one animal to train, Coleman who names his mustang, one of the more spirited ones, Marquis. The ebbs and flows in Coleman's time in the program with Marquis may mirror Coleman's own ebbs and flows in his reintegration in light of the issues he faces in dealing with various groups and individuals: the prison administration; Myles and the others in the training program; the general prison population; and the one and only visitor he currently receives, his pregnant young adult daughter Martha who has only come to clear up some administrative family issues, and thus who would have not wanted to see her father otherwise.—Huggo

Over 100,000 wild mustangs still roam across the US. Rounded up by helicopter, thousands of wild mustangs are taken from the wild to help reduce overpopulation. They are also threatened by limited resources and privatization of public land that threatens their existence.

Most spend their lives in long-term holding facilities while some are killed. Of these, a few hundred are sent to prisons to be broken and trained by inmates and later sold at auction to support the program. The helicopters isolate a group of mustangs and drive them to an enclosure where they are captured in their holding cells.

Roman Coleman (Matthias Schoenaerts) has been incarcerated for 12 years after leaving his domestic partner permanently brain damaged in an attack. A psychologist interviews Roman Coleman in a Nevada prison he's been transferred to before they reinsert him into the general population. Aware of his short temper and violent tendencies, he has resisted efforts to be reintegrated back into society. Roman is non-responsive to all questions posed to him and refuses to engage. As he isn't good with people, she assigns him to outdoor maintenance.

The first few days Roman is assigned to shovel horse feces from their pens to a dumping ground nearby. Roman notices a solitary confinement structure away from the other horses and approaches it. He is astonished to find a horse inside, who demonstrates a lot of spirit, which is why he is separated from the others. The horse won't allow anyone to touch it, and grunts loudly when Roman approaches it.

While working outdoor maintenance, Roman is seen interacting with a 'dangerous' horse. Roman is not afraid of the horse and looks it in the eye.

Roman is placed in a rehabilitation program run by rancher Myles (Bruce Dern) that assigns prisoners with training wild mustangs. Each prisoner in the program is given a specific horse to train and Roman is required to complete the training within 12 weeks before his horse is sold at an auction.

Roman has a strained relationship with his pregnant daughter Martha (Gideon Adlon), who wants his signature to sell their house and move with her boyfriend to Atlanta. Martha visits Roman in prison. At first, Roman doesn't even recognize Martha and sits with another woman. Then he is told who Martha is, and Martha then explains the reason why she came over. Roman signs the last page and tells Martha not to return.

Roman initially struggles with the training of his mustang, but under the guidance of Henry (Jason Mitchell), an inmate regarded as the top horse-trainer, Roman begins to make progress. Myles and Roman supervise the release of the mustang from its holding pen. The horse is nearly impossible to control. Henry teaches Roman to control the beast with the movements of his hand. Roman is told to remain in the center of the field and to make the horse move around him. The first part is to establish a sense of direction by getting the horse to change directions in the middle of their gallop. Never look the horse in the eye.

Martha returns to get Roman's initials on every page. Discovering she's seeking emancipation to sell the house his mother left them for money to move to the east, he refuses to sign the documents. If Roman signs, Martha would be free from restraint, control, or the power of Roman, would be free to sell the house and move in with her boyfriend, who is a mechanic. Martha says that she needs the money to raise the baby. Martha riles Roman by saying that he has no clue about taking care of a baby, as in prison his meals are served, he has a bed, and he is a dependent of the state. Roman loses his cool in front of everybody and asks Martha to leave.

The anger he feels in that situation carries over into his horse-training. Frustration with Martha causes him to lose his temper, he punches the animal repeatedly. The horse retaliates and starts to kick Roman, who is saved only when Myles intervenes. This incident causes Myles to kick him out of the horse-training program and into solitary confinement. The prison has its own system of moving good around when it is under lock-down, and Roman gets a steady stream of magazines to keep himself occupied. He reads the name Marquis in the magazine as the name of the horse of a noble.

Before a thunderstorm strikes, the horse-training inmates are woken to take the horses undercover for safety. Roman redeems himself, both getting him out of solitary and returning him to the training program. As the weeks pass, Roman becomes close with the horse, whom he names Marquis, and forms a friendship with Henry.

The program suffers a blow, however, when Henry is murdered by Dan (Josh Stewart), Roman's cellmate. Roman was stealing horse tranquilizers and bringing them to Dan, who was a drug addict. One day, Roman refused to deliver due to a falling out with Dan as Dan was demanding drugs in more and more frequency. Dan reverts by getting one of the prison gang members to cut Henry's throat. Roman retaliates by strangling Dan into unconsciousness before they are broken up by guards.

The day of the auction arrives, which Roman invites Martha to attend with the hope of improving his relationship with her. While Roman showcases Marquis to the auctioneers, he becomes distracted after he notices Martha is not present. This distraction leads to disaster when Marquis is startled by an overhead helicopter, causing Roman to fall off his horse and nearly be trampled as the other trainers restrain Marquis. Shortly afterwards, Roman learns from Myles that Marquis has been deemed not trainable and will be Euthanized. Taking advantage of the prison gates being damaged by a storm, Roman is able to help Marquis escape and return to the wild.

Some time later, Roman receives a letter from Martha acknowledging her father's refusal to leave prison. The letter also includes a photo of Martha with Roman's newly-born grandson, who she intends to have Roman meet during her next visit. After Roman finishes reading the letter, he spots Marquis standing by the prison gates and smiles.