The story opens with second-year high school student Sanada Isaki calling the school's most beautiful girl, Imiya Miori, to the rooftop after school. Isaki, burdened by a secret, is incredibly nervous, and Miori, known for her quiet and reserved nature, is visibly tense and uncomfortable. Isaki intends to ask Miori a significant favor related to his younger sister, but in his anxiety, he makes a crucial mistake, blurting out, "Please be *my* mother!" Miori, taken aback, misinterprets this as a request for a "babumi" (mother-complex) fetish, apologizes profusely, and flees the rooftop, leaving Isaki in utter despair.
The misunderstanding quickly escalates. Miori avoids Isaki, and rumors spread throughout their class that Isaki confessed to Miori, was rejected, and is now a stalker. Despite the social ostracism, Isaki is relieved that Miori hasn't revealed his embarrassing "mother" request, which he attributes to her inherent kindness. Isaki's life is already challenging; he is a dedicated older brother to his five-year-old sister, Shuri, whose mother died during childbirth. Their father is largely absent, working constantly, leaving Isaki to manage all household duties and Shuri's care. He has little time for friends or romance, his sole focus being to fulfill his late mother's last wish to protect Shuri.
The true motivation behind Isaki's request is revealed to his best friend, Matani Shinya. A few days prior, Shuri, who has never known her mother, pointed to Miori in a supermarket and called her "Okaasan" (Mom). Shuri then began asking to see "Okaasan" daily, prompting Isaki to consider if Miori could temporarily fulfill this role for his sister. Shinya, understanding Isaki's plight, arranges a second rooftop meeting to clear the air, bringing Miori's close friend, Suzuta Momone, as a mediator. Momone, fiercely protective of Miori, initially confronts Isaki as a "stalker."
During the meeting, Isaki explains his family situation, Shuri's longing for a mother, her encounter with Miori at the supermarket, and his accidental misstatement. Shinya and Momone are deeply moved by Isaki's dedication. Miori, looking apologetic, explains her silence was due to embarrassment over the "babumi" request, which she still misunderstood as a fetish, and her desire to protect Isaki's privacy. She also confesses to having noticed Isaki and Shuri at the supermarket before, admiring Isaki's brotherly care. Isaki, however, has a moment of self-doubt, wondering if he unconsciously saw his mother's image in Miori.
When Isaki reiterates his request for Miori to be Shuri's "mother" for just one day, Miori sadly refuses. She reveals her own tragic past: her mother died when Miori was two, and she never knew her father, having been raised by her maternal grandparents. She explains that she cannot be a "mother" because she doesn't know what it means or how to act in that role, lacking any personal reference. Isaki is devastated, realizing his request was unintentionally cruel. Momone, who knew Miori's past, suggests that Miori's reluctance to engage in romance stems from her parents' history. However, Momone also sees Miori's willingness to share her painful past with Isaki as a significant turning point, hinting at a "possibility" for Miori to change, though Isaki remains oblivious to the deeper implications.
