In the rugged landscapes of rural Ireland, Michael, played by Christopher Abbott, works as a shepherd alongside his father, Ray. The weight of a dark past looms over Michael; years ago, in a moment of emotional turmoil triggered by his mother's decision to leave the family, Michael caused a devastating car accident. This tragedy resulted in his mother's death and left his then-girlfriend, Caroline, portrayed by Nora-Jane Noone, severely injured. In a twist of fate, Caroline is now married to Michael's neighbor and fellow shepherd, Gary.
The tension between the two households escalates as Gary's family faces severe financial instability. After a violent storm destroys the bridge connecting their home to the main road, Gary and Caroline engage in heated arguments regarding Gary's mounting debts and his inability to fund the necessary repairs. Desperate to alleviate the financial pressure, their son Jack steals two of Michael's rams with the intention of reselling them. To cover his tracks, Jack lies to both Michael and Gary, claiming he found the rams dead on their own property and had no choice but to dispose of the carcasses.
The deception unravels when Michael discovers his missing rams for sale at a local livestock market. Consumed by rage, he confronts Gary, but Gary coldly refuses to return the animals. The situation turns volatile during the drive home, as Gary and Jack aggressively chase and taunt Michael. However, the pursuit ends abruptly when they swerve into a tree, totaling their truck. Despite their hostility, Michael rescues them and brings them home, only to be met with further ingratitude; Gary threatens him and stubbornly keeps the livestock. Inside the home, Gary orders Jack to kill the stolen rams, but Jack refuses and instead leaves with his cousin Lee. Michael chooses to keep the details of this encounter hidden from his father.
Defying his father's wishes, Michael brings his flock down from the hills to the safety of the farm. Under the cover of night, Jack and Lee steal a van and infiltrate Michael’s property. Their goal is gruesome: to steal the legs of the sheep to sell on the black market. Despite Jack's reluctance, Lee pressures him into cutting the legs from the animals while they are still alive, leaving the livestock to bleed out in agony. Michael returns to the farm after rescuing a stray, injured lamb, only to find his mutilated flock. In a desperate attempt to find the culprits, Michael chases after the departing van, mistakenly believing it belongs to Gary's hired builder. During the pursuit, Michael's dog, Mac, attacks Jack, who responds by stabbing the animal with a pocket knife. Heartbroken and desperate to end the suffering, Michael is forced to slaughter his injured sheep and his beloved dog.
Devastated, Michael reveals the horror to Ray, who reacts with a chilling command:
> "bring me the head" of the culprit.
Armed with a shotgun, Michael begins a grim vigil, stalking Gary and hiding in his pickup truck. Despite having multiple opportunities to kill him, Michael finds he cannot bring himself to shoot Gary. After Gary departs, the builder—whom Michael believes is the perpetrator—spots him and attempts to flee. A violent struggle ensues, resulting in the builder shooting Michael in the ear. In an act of self-defense, Michael pushes the man into a pile of bricks; the falling debris kills the builder instantly. Driven by his father's demand, Michael beheads the corpse and packs the head into a gravel bag.
Bloody and wounded, Michael makes his way to Gary's house, where Caroline tends to his injuries. While helping him, Caroline notices a knife Michael recovered from his dog and recognizes it as belonging to Jack. This realization strikes Michael with the truth: the real culprit was not the builder, but Jack. Caroline and Jack drive Michael back to his home, carrying the bagged head without their knowledge. As they pass the site of the slaughtered sheep, Caroline is horrified by the carnage.
Upon arriving, they find Ray collapsed on the ground, seemingly unconscious. When Caroline finds she has no phone signal to call for help, she decides to leave to find a doctor, leaving Jack alone with Michael and Ray. Ray awakens and opens the bag to reveal the severed head. Michael tells his father that he has fulfilled the request, though he admits it is the wrong head.
Terrified by the madness, Jack flees up the mountain, pursued by a harrowing Michael. When Michael finally catches him, Jack manages to trick and stab him, though he offers an apology for his crimes. During a desperate struggle over the knife, Jack accidentally stabs himself. In a final act of tenderness, Michael comforts the dying boy and carries him back home. There, Caroline confronts them with a rifle after discovering the severed head. Michael uses this moment to finally apologize to Caroline for the car accident from years ago. Softened, she puts the gun down and drives the injured Jack away. Left alone and bleeding, a dazed Michael slowly steps back into his home.
