Mirai Kuriyama is the last remaining member of a Spirit World warrior clan, possessing the unique ability to use her own blood as a weapon. Because of this, she is assigned to eliminate "youmu," creatures that are born from humanity's negative emotions. While lost in thought on the school rooftop one day, Mirai encounters Akihito Kanbara, a rare half-youmu who appears human. Startled, she stabs him with her blood blade, only to discover that he is immortal. From that moment, the two develop an unlikely friendship, with Mirai repeatedly trying to kill Akihito as a way to build her own shaky confidence as a Spirit World warrior. Eventually, Akihito persuades her to join the Literary Club, which includes two other formidable Spirit World warriors, Hiroomi and Mitsuki Nase.
As the group grows closer, however, the youmu around them become more aggressive. Their lighthearted adventures soon escalate into a struggle for survival, as the release of the most powerful youmu, Beyond the Boundary, draws near.