Set prior to the main series, episode zero delves into Hiroomi Nase's task of confronting the enigmatic half-human, half-youmu Akihito Kanbara. Intrigued by her brother's assignment, Mitsuki Nase joins him, leading to their initial encounter with Akihito. Yet, as they delve deeper, they realize they are not the sole pursuers of Akihito, setting the stage for the unfolding tale of Kyoukai no Kanata.