A former Marine competes against a violent drug dealer in a bare-knuckle fight, for a large cash prize. Jarhead Earl (Jamie Bell) is a former marine and lives with his wife and 2 kids. His wife Sarah (Valerie Jane Parker) is a druggie and gets her hits from Chainsaw Angus (Frank Grillo) who is a drug dealer. Earl gets into a fight with Angus to kick him out of his house, but is defeated when Angus is helped by his sister Delia (Margaret Qualley). Angus beats up Delia as she failed to warn him about Earl's entrance into the house.Angus and Delia approach Eldon (Pat Healy) and ask for his help to front the money to enter the fight. But Eldon owes money to his own creditors and refuses to help. Angus beats up Eldon, badly. Delia ties him up, has sex with him and when he is about to cum, she shoots him dead. Meanwhile earl is robbing small businesses cash registers to keep his house going. He uproots his own family and shifts into a hotel. He goes with his son to run errands, when his car dies, so he steals a cop car to keep moving.Whalen (James Badge Dale) is the sheriff of the county where Angus and Earl are operating. In a fit of rage and angered by his constant put downs, Delia shoots Angus, but he survives. Earl finds practice fights to prep for the big event, but eventually his stolen cop car is detected and he is back on the run with his son. His fight fixer Poe (James Landry Hébert) wants to take him to big fight called the DonnyBrook, but wants Earl to find a place to keep his son. Meanwhile Delia catches up with Earl at the practice fight. Poe suggests an old couple who can care for Earl's son while they all head to Donnybrook.But now Angus is on Delia's trail and finds Poe. Whalen tracks down Angus, but is shot by him. Angus is now on Delia'a trail who is with Earl and his son. Angus finds Earl's son cycling in the woods and shoots him dead. But by then Earl and Delia are already on their way to Donnybrook. Angus finds Delia at the Donnybrook, takes her into the woods and strangles her. He then enters the ring himself. After the prelim rounds when only Angus and Earl are left, Angus gives Earl his son's locket, telling him that he killed him. After a brutal fight, Earl kills Angus in the ring.