Nagisa Natsunagi has finally achieved what she once thought impossible: a normal high school life. After years of illness kept her away from school, a successful heart transplant allowed her to return to the classroom, and with the help of her two best friends, she is living out her quiet dream of being just another ordinary student. But her new heart brings with it an unshakeable curiosity about the donor who saved her life, and a rumor spreads through town about a boy who can solve any case. That rumor drives Nagisa and her friends to investigate a strange incident at school, setting off a chain of events that pulls her away from the everyday world she worked so hard to enter.

Nagisa Natsunagi is the central character and first-person narrator of the story. She is a high school girl who, despite her frail medical history, carries a resilient and earnest desire to experience the mundane joys of youth. Her two unnamed best friends support her return to school life and join her in investigating the local mystery. The boy she eventually finds—the rumored detective—becomes a catalyst that pushes Nagisa into a summer of adventure far beyond the classroom.

The setting shifts between a contemporary Japanese town and the school where Nagisa attends classes. The atmosphere evokes a gentle, slice-of-life tone that gradually opens into a wider, more mysterious world as Nagisa follows the detective’s trail.

In the first volume, Nagisa reenters school, grapples with the lingering awareness of her donor’s presence inside her, and begins her search. By the end, she locates the detective, and the quiet life she cherished begins to fray. The second volume picks up after that discovery. Nagisa finds herself caught in a whirlwind summer filled with new encounters and adventures that test her sense of belonging. She worries that her involvement in this strange world has created distance between herself and her friends, but they welcome her back without reservation. Just as she dares to hope for a return to normalcy, a silver-haired junior student who bears a striking resemblance to the detective appears and abruptly confesses to her, throwing Nagisa’s heart and her ordinary days into fresh turmoil.

The series serves as a spin-off focused on a supporting character from the main The Detective Is Already Dead series, reimagining her story from a grounded, everyday perspective. Its narrative arcs balance the warmth of friendship and the fragility of second chances with the quiet mystery of a past life that refuses to stay buried.