A girl named Rei and her little sister Mirai are pulled into the afterworld of Purgatory by an unknown force. Frightened and lost, Rei tries to lead them home but is attacked by a monstrous threat. To protect Mirai, Rei unlocks a hidden power and battles the restless Souls of Purgatory, but tragically mistakes her sister for an enemy and kills her. Overcome with despair, Rei weeps, and two Demons offer her a deal: if she hunts wandering Souls and delivers tears of suffering, they will bring Mirai back to life. Accepting their contract, Rei becomes an Executor of Purgatory, embarking on a perilous journey filled with relentless battles—all to save the sister she loves.