Mirai Kuriyama is the last survivor of a Spirit World warrior clan with the ability to use their blood as a weapon. As a result, Mirai is entrusted with tracking down and eliminating "youmu," which are thought to be the embodiment of unpleasant human emotions. Mirai stumbles finds Akihito Kanbara, a rare half-breed of youmu in human form, one day while deep in meditation on the school roof. In a panic, she thrusts her blood blade into him, only to discover that he is an eternal entity. From then on, the two create an unplanned relationship centered on Mirai's repeated attempts to murder Akihito in order to strengthen her own shaky confidence as a Spirit World fighter. Eventually, Akihito persuades her to join as well.