Hojoong, the Indomitable Champion, was at the peak of his career as a mixed martial arts fighter when a shoulder injury forced him into early retirement. He then spiraled into a life of addiction and gambling, eventually hitting rock bottom. Burdened with a debt of 700 million won, his wife, who had been managing the interest payments, finally gives up and leaves with their son. When his creditors threaten to take his organs as payment, Hojoong encounters a mysterious stranger. The stranger offers Hojoong a chance to participate in the "Brutal Cup", the largest underground fighting competition. Unaware of what awaits him, Hojoong accepts the offer.