Humanity and demonkind have been on bad terms for millennia, resulting in the continent of Leunders being split in two. Although the situation has stabilized between the races, the fragile peace breaks when the new demon king launches a fierce all-out attack. With all of the non-demon nations falling to the violence in under a year, humanity's hope rests in a special squad, led by swordsman Kyle Leonard, that must assassinate the demon king in order to bring an end to the war.

During the dangerous mission, Kyle ends up in a clash with the demon king that brings him to the brink of death. As Kyle realizes he has lost everything he wanted to protect, a mysterious gem transports him back in time before the conflict even started! Now with knowledge of future events and a strong desire to save his loved ones, Kyle sets out to become even stronger to prevent the extinction of humanity.