As Nero was being crowned emperor of the Roman Empire at the tender age of 17, Cestvs the slave was fighting in a boxing match in order to become a fighting slave. However, the moment he defeats his friend from the training school, countless arrows strike his friend in the back. Defeat means death... Cestvs is terrified to learn the cruel fate of fighting slaves. He is promised his freedom if he wins one hundred matches, but the path to freedom is long. A competition between training schools sponsored by the emperor is to be held in Rome, and Cestvs steps onto the big stage.

During an audience with Emperor Nero, Agrippina orders Cestvs to fight the young guard Ruska. After being choked out by fighting prodigy Ruska, Cestvs is discouraged by his first defeat, but when Zafar tells him that he has a future, he pledges to become stronger. Time passes, and Cestvs visits Pompeii as part of Drisco's boxing group, where he encounters Pompeii's strongest fighting slave, Emden. At the mansion he is invited to, he spots a person sneaking in late at night... who turns out to be a beautiful woman. Who is she really?!

Sabina, daughter of a wealthy family of Pompeii, orders Cestvs to fight some thugs in the city. After realizing his own powerlessness, he asks Zafar to teach him how to throw a single knockout punch. Sabina offers to purchase Cestvs, but he rejects her offer, saying he wants to buy himself back on his own. Offended by his rejection, Sabina instructs Emden to thoroughly beat him in a boxing match. Emden agrees. He and Sabina have made a promise that will be fulfilled once he completes his undefeated record.

Cestvs' fight against Emden, who is aiming to achieve an undefeated winning streak, has begun. Cestvs struggles with Emden's strength, which is even greater than he imagined. Despite it, he attacks by striking swiftly, but his opponent throws an unexpected series of blows that down Cestvs. Cestvs gets back up on unsteady legs and goes for a counterattack, but rather than attacking back, Emden focuses on defense. However, the look in his eyes reveals that he is up to something. Just as Cestvs attempts to land a powerful single blow, Emden's terrifying objective becomes apparent.

Emden's Decapitator is a literal killer technique. Cestvs' muddy consciousness recalls the single knockout blow his master Zafar taught him. Furthermore, Emden has been injured by Cestvs' attacks so far. He could win by trading blows... But in the next moment, Emden's fist cuts Cestvs' brow, which begins bleeding into his eye. It's kill or be killed. While one of Cestvs' eyes is blinded, Emden attempts to finish him off.