Once upon a time, our protagonist and Sanemi Shinazugawa were lovers, enjoying a happy and peaceful life together. However, one tragic day, Sanemi was attacked by a demon and turned into one himself. With his transformation, Sanemi's personality and behavior changed completely, becoming cold and violent, no longer recognizing his lover. The protagonist is devastated and heartbroken by the loss of Sanemi to the demon's curse, and they struggle to come to terms with the fact that the person they loved has been replaced by something monstrous and dangerous.

Despite their heartbreak, the protagonist refuses to give up hope that there is still a chance to save Sanemi and free him from the curse of demonhood. They start searching for a way to revert Sanemi back to his old self, consulting with other demon slayers and exploring every possible avenue to find a cure. In the meantime, they must also keep themselves safe from Sanemi's attacks, as the demon in him has no qualms about harming the ones he once loved.