In "Beyond the Boundary," Mirai Kuriyama, the sole survivor of a Spirit World warrior clan, wields a unique power to transform her own blood into a lethal weapon. Her duty is to eradicate "youmu," entities born from human negativity. While lost in thought on a school rooftop, she unexpectedly crosses paths with Akihito Kanbara, a rare half-youmu who appears entirely human. Startled, she strikes him with her blood blade, only to realize he is immortal. Despite this, an unusual bond develops, and Mirai repeatedly attempts to kill Akihito in an effort to bolster her wavering confidence as a Spirit World warrior. Eventually, Akihito convinces her to join the Literature Club, where she meets Hiroomi and Mitsuki Nase, fellow skilled warriors from the Spirit World. As their friendship deepens, the surrounding youmu grow increasingly hostile. What begins as lighthearted adventures evolves into a desperate fight for survival when the most powerful youmu, known as Beyond the Boundary, is on the verge of being unleashed.