Doug Glatt (Seann William Scott) from Massachusetts, feels ostracized from his family, his father (Eugene Levy) and brother both being doctors. One day he attends a minor league hockey game with his friend Pat (Jay Baruchel). Pat taunts the visiting team during a fight and one of their players climbs into the stands. Doug, in defense of his friend, quickly knocks him out, which prompts the rest of the crowd to cheer him on. Soon after, Doug gets a phone call from the coach of his hometown team who offers him a job as an enforcer, a player whose role is to protect his teammates and act as a deterrent by hitting or fighting opposing players who take liberties with his teammates.
In the meantime, veteran enforcer and Doug's idol Ross "The Boss" Rhea (Liev Schreiber) is demoted to the minors after serving a 20 game suspension for slashing an opponent in the head from behind. Three years prior, Rhea hit and concussed the highly skilled prospect Xavier Laflamme (Marc-Andre Grondin) who has had trouble recovering from that incident due to the fear of being hit, being stuck in the minors and falling in with the wrong crowd. After earning himself the nickname "The Thug", Doug is called up to Canada and hired by Laflamme's team, the Halifax Highlanders, to protect Laflamme and be his roommate.
The Highlanders experience success with Doug as their primary enforcer, and he quickly gains popularity among fans and teammates much to the chagrin of his parents and Laflamme, particularly after losing ice time and the alternate-captaincy to Doug. Doug becomes romantically involved with Eva (Alison Pill), a hockey fan with a penchant for players.
With 4 games left on their schedule, the Highlanders need two wins to secure a playoff spot. On a road game in Quebec, after an opposing player concusses Laflamme with a heavy hit, Doug savagely beats the player unconscious and is suspended for the next game against Rhea and the St. John's Shamrocks. Doug encounters Rhea at a diner, where Rhea dismisses Doug's claim that he is a hockey player, calling him a goon. Rhea warns him that if they ever meet on the ice, he will "lay him the fuck out." The Highlanders, with Doug suspended and Laflamme hospitalized, lose to the Shamrocks.
Doug reaches out to Laflamme, and promises him he will always have his back on the ice. In their next game, the Highlanders lead 1-0 thanks to renewed teamwork between Doug and Laflamme. In the dying seconds, Doug blocks a slapshot with his face and his ankle is broken in the ensuing scramble. The Highlanders win, but need a win against Rhea and the Shamrocks in their last game for a playoff spot.
After two periods, the Shamrocks are beating the Highlanders 2-0. Rhea and Doug drop the gloves in the third period, and dole out and receive physical punishment during the fight. Doug is knocked down first, but Rhea calls off the referees and allows him to get back up. Doug manages to break Rhea's nose, but breaks his previously injured ankle in the process. Doug manages to stand back up and knocks out Rhea with a vicious cross. Eva and his teammates help a seriously injured Doug off the ice and Laflamme, inspired by Doug's efforts and Rhea's demise, scores a hat-trick to lead the Highlanders to a 3-2 victory and a play-off berth. While being comforted by Eva in the locker room, Doug victoriously comments, "I think I nailed him."