Mirai Kuriyama is the only survivor of a clan of Spirit World warriors who possess the ability to use their blood as weapons. Consequently, Mirai is assigned the duty of tracking down and eliminating "youmu," entities described as manifestations of negative human emotions. One day, while lost in thought on the school roof, Mirai encounters Akihito Kanbara, a rare half-breed youmu in human form. Acting in a panic, she thrusts her blood saber into him, only to discover that he is an immortal being. From that point forward, the two develop an impromptu friendship centered around Mirai's persistent attempts to kill Akihito, a practice she uses to bolster her own fluctuating confidence as a Spirit World warrior. Eventually, Akihito also succeeds in persuading her to join the Literary Club, which includes two other powerful Spirit World warriors, Hiroomi and Mitsuki Nase. As the group's bond grows stronger, however, the tenacity of the youmu surrounding them also increases. Their misadventures will soon escalate into a struggle for survival as the inevitable release of the most powerful youmu, Beyond the Boundary, draws near.