Saito Yuichi, a young man with no ambition or prospects, decides to end his life on New Year’s Eve. At the very moment he intends to leap, he meets Mitsumune Sakutaro, a charismatic and composed stranger who is Yuichi’s complete opposite. That meeting does not give Yuichi a reason to live—it swaps their bodies and their fates. Yuichi awakens in Sakutaro’s body and soon discovers that Sakutaro is a prolific serial killer. Meanwhile, Sakutaro, now in Yuichi’s frail body, begins to manipulate events from the sidelines. Forced to navigate a world of murder, police investigations, and Sakutaro’s criminal network, Yuichi must pretend to be the killer while desperately seeking a way to reverse the exchange. The story follows his struggle to survive, the unexpected bonds he forms with a small girl and other misfits drawn into the chaos, and the psychological warfare waged between two men who are each other’s mirror. As the police close in and the body count rises, Yuichi is confronted with a horrifying question: when you become the monster, how do you prove you are not?