In a society where humans and demi-humans coexist under a rigid classification system, four categories separate people: hominids (full humans), subspecies (demi-humans with overt traits), and two intermediate groupings. Tamaki Yuika, a hominid girl, has been best friends since childhood with Yukimi, a subspecies girl. Despite the invisible walls that their world insists on, the two pledge to attend the same high school and become classmates. Their bond is shattered by a sudden incident: physical contact between them triggers a harmful reaction, and Yukimi later attempts suicide, believing that coexistence is impossible. The trauma drives Yuika into a deep depression, but she resolves to prove her friend wrong.

Yuika transfers to a new high school in a large city, where the rules are different—hominids, subspecies, and all other categories mix freely in the same classrooms. There she meets a cast of new classmates: Katsuha, a bashful kappa girl who hides her bald head under a floppy hat; Saku, a quiet girl who conceals Medusa-like snakes under her hood; and Anna, a gloomy cat-eared girl who shares Yukimi’s despair about the divisions. Each new friendship challenges Yuika to understand the pain of being different and to find her own way of bridging the gap between categories.

The story follows Yuika’s growth through episodic encounters with various demi-humans, from a shy oni to a bird-like student, as she slowly rebuilds her belief that connection across species is possible. Her journey is both a personal quest to honor her promise to Yukimi and a broader exploration of what it means to live in a world that insists on sorting people into boxes. Through small acts of kindness, misunderstandings, and heartfelt conversations, Yuika learns that the line between “human” and “subspecies” is less important than the bonds they choose to form.