The film follows the harrowing journey of Richard (Paddy Considine), who returns to his hometown of Matlock, Derbyshire, after a stint as a paratrooper in the British Army. Accompanied by his younger brother Anthony (Toby Kebbell), who suffers from a mental impairment, they find refuge at an abandoned farm nearby. The film unfolds through flashbacks that expose the traumatic experiences Anthony faced at the hands of local drug dealers, prompting Richard's thirst for revenge.
In a tense confrontation, Richard dons a mask and confronts Herbie (Stuart Wolfenden), one of the abusers, who fails to recognize him initially. Later on, as Herbie and his cohorts Soz (Neil Bell) and Tuff (Paul Sadot) indulge in drugs at their flat, they discuss the masked intruder, with Herbie suspecting it could be Richard, Anthony's brother. When Herbie steps outside, he catches a glimpse of a military-gas-masked figure banging on the door, but the mysterious figure vanishes. Upon returning inside, they discover their place has been ransacked, drugs stolen, and the ominous words "Cheyne Stoking" spray-painted on the wall—a reference to the dying breath pattern of a human.
The following day, the gang approaches Sonny (Gary Stretch), their de facto leader, to explain their predicament. Unbeknownst to them, Sonny’s gang has also had a makeover, painting their faces, hair, and clothes. Tensions rise, especially when they realize the man from the pool hall is indeed Richard, igniting a sense of dread as they acknowledge his return.
The encounter escalates when they eventually cross paths with Richard, who boldly invites them to confront him at the farm. The gang, shaken by his fearlessness, decides to track him down. That night, Richard stealthily infiltrates their hideout, leading to a brutal murder of one gang member with an ax, leaving the chilling message "One Down" smeared in blood on the wall.
Panicked, Sonny plots to eliminate Richard. The next morning, they plan to lure him out and execute their scheme, but in a tragic twist, Sonny accidentally shoots one of his own men, Big Al (Seamus O'Neil), instead of Richard. With their plan in shambles, they retreat, leaving Richard smiling, full of confidence.
As the surviving gang members regroup at a petrol station, tensions heighten, and Tuff scampers off in fear. Later, the gang arm themselves in Sonny's home, but while they search upstairs, Richard slips into the kitchen pantry. In a bid for revenge, he poisons their kettle with the drugs he previously confiscated, leading to a chaotic scene where he takes pleasure in tormenting and killing them one by one.
After dispatching Sonny and Soz, Richard confronts Herbie, presenting him with Tuff's dead body and forcing him to divulge the whereabouts of the final gang member. After a series of cruel interactions, Richard allows himself to be killed when he binds Mark (Paul Hurstfield), the last gang member, to a confrontation over their shared past. Ultimately, it’s revealed that Richard has been alone all along, haunted by the memory of his brother.
As the emotional climax unfolds, Richard takes Mark to the very location where Anthony's tragic death unfolded, confronting the painful truth of his brother's torment. In a tragic resolution, Mark stabs Richard, ending his cycle of vengeance and despair.
