Arm, Cosmo Jarvis is a retired boxer who now works as muscle for the Devers crime family after accidentally killing an opponent in the ring. Under orders from his close friend and boss Dympna Devers, Arm brutalizes a Devers associate named Fannigan for drunkenly trying to climb into bed with Dympna's youngest sister, Charlie, who is only 14, though Arm insists he did not intend to molest her.
Arm visits his estranged partner Ursula and their autistic son Jack, who both live with his mother. Although Ursula and Arm still harbor feelings for one another, the Devers family remains dangerous, and Arm struggles to cope with Jack's outbursts and needs. Ursula has been taking Jack to therapeutic horseback riding sessions at a local farm and is dating the therapist Rob. She has enrolled Jack in a special needs school in Cork and asks Arm to help pay the tuition.
Dympna Devers and Arm are summoned to the Devers farm, where Dympna's uncles Paudi and Hector run the family business. Hector says he heard about Charlie and admonishes the pair for letting Fannigan live, giving them one chance to kill Fannigan themselves. Paudi subtly threatens to kill Jack if Arm cannot follow through on these orders. Dympna gets Arm high, and tells him to push Fannigan off the edge of the seaside cliff. Arm prepares to strike him with a stone and cast him naked into the sea, making it look like a suicide, but cannot bring himself to do it. Instead, he tells Fannigan to leave town and never return. Arm tells Dympna he followed through on the murder, and Dympna tells his uncles.
Arm starts visiting Jack when he is with the horses and offers to take more responsibility for Jack, impressing Ursula. However, Jack becomes overstimulated at a fair under Arm's care and has an episode, causing Arm to panic and return Jack to his grandmother. Ursula tries confronting Arm at Dympna's house, but the Devers accuse her of abusing drugs while pregnant with Jack and using Arm for his money. Arm promises to make things right with Ursula and Jack and to provide money for the school in Cork, but urges Ursula to stay away from the Devers in the meantime.
Dympna Devers and Arm are again called to the farm, this time to discuss Fannigan's murder with Paudi. Arm maintains his lie that he killed the man, but Paudi reveals Fannigan's corpse in a barn, evidently tortured to death. Paudi orders Dympna to kill Arm for his disloyalty, but Dympna allows Arm to flee for his life. Arm successfully escapes, but is shot in the side by Paudi. Arm makes his way to the house of Maire, a wealthy widow Hector has been seeing, and attempts to rob her so Jack and Ursula can get to Cork and escape the Devers' wrath.
Dympna calls Arm's cell phone with Paudi beside him to try and call him back to the farm with threats to Jack and Ursula. Arm takes Hector hostage and tells Paudi to meet him at Maire's house. Hector escapes in Arm's car, and Maire tries convincing Arm to see a doctor for his gunshot wounds. Recognizing Arm as the boxer who accidentally killed his opponent, Maire tells him his loved ones would not want this for him. He allows her to leave with her savings, but stays himself to await Paudi. Ursula calls Arm to announce she got a job at the special education school in Cork, which will pay for most of Jack's fees. Arm apologizes for acting so immaturely and tells Ursula he loves her. As Paudi pulls into the widow's driveway, Arm ends the call and waits in a chair with a hammer, bleeding out rapidly. The final scene sees Ursula packing to move to Cork, then walking Jack to his new school.
