Arm is a retired boxer who now works as muscle for the Devers crime family after an in-ring tragedy. Under the orders of his close friend and boss Dympna Devers Barry Keoghan, Arm Cosmo Jarvis must enforce the family’s ruthless code. When a Devers associate named Fannigan Liam Carney is accused of drunkenly trying to climb into bed with Dympna’s youngest sister, Charlie Hazel Doupe he is beaten as punishment, a chilling reminder of how tightly the Devers grip their world, and how fragile Arm’s own loyalties have become.
Arm later visits his estranged partner Ursula Niamh Algar and their autistic son Jack, who live with Ursula’s mother. Although the pair still harbors feelings for one another, the Devers blood-soaked business creates constant danger, and Arm struggles to manage Jack’s outbursts and needs. Ursula is moving forward, taking Jack to horseback therapy sessions at a local farm and dating the therapist Rob Anthony Welsh. She has also enrolled Jack in a special needs school in Cork and asks Arm to help cover the tuition, a practical line that tugs at their lingering connection.
Dympna and Arm are summoned back to the Devers farm, where Hector David Wilmot and Paudi Ned Dennehy run the family business. Hector voices concern after hearing about Charlie, and he gives the pair one last chance to deal with Fannigan themselves. Paudi hints that he could harm Jack if Arm cannot fulfill the orders, tightening the pressure. High on drink and power, Dympna pushes Arm toward a brutal option: push Fannigan off a seaside cliff and stage a suicide. Arm steadies himself, lifting a stone and considering a naked throw into the sea, but in a moment of hesitation he cannot complete the act and instead tells Fannigan to leave town for good. He later confesses to Dympna that he followed through on a murder, a claim that unsettles the uncles and the entire predicament.
Arm begins to visit Jack by the stables, offering to shoulder more responsibility and earning Ursula’s cautious trust. Yet a fair outing turns into a nightmare when Jack becomes overstimulated; Arm panics and returns Jack to his grandmother, an incident that strains Ursula’s trust further. Ursula confronts Arm at Dympna’s house, but the Devers respond by accusing her of drug use while pregnant and of exploiting Arm for money. Arm promises to make amends—to repair the relationship with Ursula and Jack and to fund the Cork school—but urges Ursula to stay away from the Devers’ orbit for the time being.
The tension escalates as Dympna and Arm return to the farm to discuss Fannigan’s murder with Paudi. Arm maintains the lie that he killed the man, while Paudi reveals Fannigan’s body in a barn, tortured and dead. Paudi orders Dympna to kill Arm for his treachery, yet Dympna spares him, allowing Arm to flee. A gunshot wounds Arm in the side as he escapes, and he makes a desperate bid for safety by heading to Maire’s house—the widow Hector has been seeing—in the hope of robbing her so that Jack and Ursula can flee to Cork.
Dympna calls Arm, Paudi listening in, threatening to target Jack and Ursula if Arm does not return. Arm seizes Hector’s car and holds Maire hostage, forcing Paudi to meet him at Maire’s house. Hector escapes in Arm’s car, and Maire urges Arm to seek medical help for his wound, while reminding him that his loved ones would not want this life for him. He refuses to surrender Maire’s savings, letting her leave with them, and waits to confront Paudi. Ursula phones again with news that she has secured a job at the Cork school, a lifeline that would finance most of Jack’s tuition. Arm apologizes for the reckless behavior and tells Ursula he loves her, but then ends the call as Paudi arrives at Maire’s driveway. Arm sits in a chair, hammer in hand, bleeding and waiting for the end. In the end, Ursula packs to move to Cork and walks Jack toward his new school, a somber reminder of the costs of the men’s code and the fragile ties that hold the family together.
