Itou Yuki is a high school girl crippled by social anxiety, often called a monster behind her back for how awkwardly she mimics human interaction. She envies her classmate Takahashi Ayaka, who seems effortlessly friendly and well-liked. One day, Itou stumbles upon Takahashi in the act of swallowing a person whole. Rather than recoiling in horror, Itou is awestruck: a real monster has perfectly blended into human society, and she wants that same ability more than anything. She resolves to become Takahashi’s friend.

Takahashi, a creature that has long learned to mimic human behavior in order to survive, agrees to teach Itou the ropes of “normal” social conduct. Under Takahashi’s guidance, Itou begins to navigate everyday school life with newfound confidence, but the arrangement is not one-sided. Takahashi has her own hidden agenda, and the true cost of this mentorship slowly comes into focus as other monsters begin to appear.

The story unfolds in a contemporary Japanese high school, where the ordinary routines of classes and friendships collide with the predatory, shape-shifting creatures that have infiltrated human society. A key narrative arc involves a former teacher, the late 贄原先生, who returns as a monster and proposes that Itou help him wipe out humanity. Itou must grapple with the moral weight of that offer while protecting the fragile normalcy she has worked so hard to achieve. Tensions escalate when another monster kills and impersonates a classmate named 平泉さん, leaving Itou torn between preserving the peace of ignorance and seeking justice for the murdered girl. Takahashi quietly reminds her that if she truly wants to end the monsters, she will have to kill Takahashi herself at the end.

The manga traces Itou’s transformation from a withdrawn outsider into someone willing to fight for the world she has come to care about, even as the line between human and monster blurs. It concludes with a confrontation that forces Itou to decide what kind of “normal” she is willing to accept, and at what price.