Sixteen-year-old Hinata Asahi, known to her friends as Hina, lives alone in a large house, accompanied only by her two beloved teddy bears, Waka and Tono. Her father, the wealthy chairman of a school board, is constantly traveling for work. Her solitary life takes an unexpected turn when a new neighbor moves into the house next door. The man is Shiro Asou, a seemingly cold and indifferent 32-year-old who, the next day, is revealed to be the new substitute teacher at Hina's high school.

What initially appears to be the setup for a platonic and complicated student-teacher romance soon unravels into a story steeped in mystery. Shiro is not just a teacher; he is a former secret service agent and a bodyguard, secretly hired by Hina's father. Having been kidnapped nine times before due to her father's wealth, Hina is in constant danger, and Shiro's mission is to protect her from a potential tenth abduction. As Hina develops deeper feelings for her reserved teacher, she remains unaware that her house is wired with cameras and microphones for her protection. Shiro, haunted by a past mission where a client he fell in love with was shot while saving him, struggles to suppress his own growing affection for Hina, believing his feelings would put her in danger.

The central conflict forces Hina out of her innocent and childlike worldview as she confronts the sinister reasons behind Shiro's presence. This tumultuous period of maturation is further complicated by several key events. Hina’s classmate, Emi Kishitani, whose father is in debt to yakuza loan sharks, hatches a desperate plan to kidnap Hina for ransom. After Shiro rescues her, Hina's profound loyalty leads her to forgive Emi and help her family, deepening their friendship. Another crucial figure is Tomoaki Namiya, a 21-year-old author of children's books whom Hina admires. Tomoaki, who also has a bodyguard named Masaya Kizu, begins writing a novel titled Suki that eerily mirrors the real-life relationship between Hina and Shiro, adding a layer of meta-narrative to their journey. The story follows Hina as she navigates these betrayals and threats, slowly chipping away at Shiro’s emotional walls while proving her own strength is greater than her naïve exterior suggests. The narrative concludes with their eventual reunion and the establishment of a formal romantic relationship, which is depicted in later CLAMP works such as Kobato.