Some individuals possess the ability to enter other people's minds and control their memories. This power is employed to eliminate mysteries and cases, or, in worse scenarios, to commit assassinations. The potential of this ability is potent enough to destroy a person's mind, yet it can backfire and consume the user's own heart. To counter this, chains are utilized to lock and protect each other's vulnerable and dangerous hearts. Due to growing fear and disdain, they are consequently referred to as "Pet."