A melancholy U.S. Marine Thomas "Tommy" Riordan visits his father, Paddy Conlon, a recovering alcoholic who has returned to his Catholic faith in Pittsburgh, Pennsylvania. As a child, Tommy had to run away from Paddy with his dying mother due to Paddy being an abusive alcoholic, and has never forgiven him. Paddy tries to convince him that he has changed, but to no avail. The next day, Tommy enters a gym where he quickly knocks out Pete "Mad Dog" Grimes, one of the world's best middleweight fighter; a video of the fight goes viral on the Internet. Learning about a winner-takes-all mixed martial arts tournament called Sparta, whose winner will receive $5 million, Tommy asks Paddy to help him train for the tournament, but only under the condition that Paddy does not try to reconcile their relationship. Tommy pledges to win and then donate the entire $5 million purse to Pilar, the widow of one of his friends in the corps.
