The story unfolds through the eyes of Hashimoto Aqua, a high school freshman who initially finds his classmate, Fujiwara Sana, peculiar and annoying. Fujiwara, a small girl with a childlike face, exhibits strange behaviors from the outset, such as abruptly leaving the entrance ceremony and asking Hashimoto's name in an unconventional manner, even calling his given name "Aqua" a "kirakira name." Despite her social awkwardness and isolation, she possesses an encyclopedic knowledge, which she often shares in long, unsolicited monologues, detailing everything from school building layouts to complex scientific theories. Hashimoto's friend, Noda, often shares his exasperation with Fujiwara.
As the first month progresses, Hashimoto and Noda join the soccer club. Fujiwara, surprisingly, joins both the literature and volleyball clubs, despite her small stature and apparent social difficulties. Hashimoto observes her unique academic profile: struggling with basic questions but excelling in complex physics and math proofs. He also notices the growing animosity from classmates and teachers, who often ignore her or react negatively to her eccentricities.
A turning point occurs during a seat change when Hashimoto witnesses a classmate, Yamamoto, roughly shove Fujiwara's hand. Hashimoto, conflicted, does nothing, leading to self-loathing. This incident, coupled with Fujiwara's occasional fleeting expressions of sadness, sparks a shift in Hashimoto. He begins to research neurodevelopmental disorders, suspecting Fujiwara might have one, and realizes he is becoming "obsessed" with understanding or protecting her. He exchanges LINE IDs with her, helping her with homework for teachers who dislike her, and recalls a humiliating classroom incident where Fujiwara debated an English teacher about pronunciation.
During cultural festival preparations, Fujiwara's hidden talents for art and design are revealed, earning her praise from the class representative, Miyano. However, Hashimoto makes a disturbing discovery: large, dark bruises covering Fujiwara's back. He suspects abuse, either at home or from the volleyball club, specifically targeting a large, unpleasant girl named Ikebuchi. He tries to keep Fujiwara close to protect her.
On the first day of the cultural festival, Hashimoto works at the successful butler cafe. Later, while exploring the festival with Noda and the soccer club managers, including the attentive Terasaka, they discover Fujiwara playing a beautiful piano piece dressed as a ninja. Her skill is remarkable, but Terasaka mentions Fujiwara's reputation for causing problems in the volleyball club. On the second day, after serving Hashimoto and his friends at the cafe, Fujiwara declines to attend the after-party. Later that night, Terasaka confesses her feelings to Hashimoto, but he coldly rejects her, asking her to stop calling him "Aqua-kun."
The rejection has immediate social repercussions for Hashimoto. Terasaka spreads the news, leading to his isolation from the soccer club managers. Meanwhile, Hashimoto confronts Fujiwara about her bruises, but she denies any abuse, stating she's used to being disliked and warns him not to involve teachers. His frustration mounts when Fujiwara reveals she will start taking Concerta for her ADHD, explaining it will suppress her "extra behaviors" and help her focus on studies. Hashimoto is deeply disturbed by the idea of medication altering her personality and causing insomnia. His protective instincts clash with his frustration over her evasiveness, culminating in an emotional outburst where he yells at her. Fujiwara flinches, her eyes welling up, leaving Hashimoto consumed by guilt and a deepening sense of gloom. He feels he has failed to protect the very person he wanted to shield from harm.
