For as long as she can remember, Sunao Aikawa has had the ability to summon a perfect copy of herself. She created this replica, whom she calls Nao, when she was seven years old to handle tasks she wanted to avoid. Now in high school, Sunao routinely sends Nao to take her place on days she feels sick or simply does not want to get out of bed, forcing her replica to attend classes, take exams, and endure everything Sunao wishes to escape. Nao accepts this existence without complaint, believing her only purpose is to be useful to her original. She sleeps on the floor, owns nothing of her own, and can be dismissed into nothingness with a single command.

The story takes place in an otherwise ordinary Japanese seaside town, where the magical realism of the replica system goes unquestioned. Nao finds small moments of joy in reading and spending time in the literature club, and she begins calling herself Nao as a small act of claiming her own identity. Her world changes when a boy named Shuuya Sanada returns to school after recovering from a leg injury. He notices that Nao acts differently from the usual Sunao, becoming the first person to see her as someone distinct. To help him tell them apart, Nao asks him to speak with her only when she wears her hair in a half-ponytail. As they grow closer through their shared time in the literature club, Nao realizes she has developed romantic feelings for him, which forces her to confront a painful question: can a replica, someone who technically owns nothing and exists only at anothers pleasure, truly fall in love?

Shuuya eventually reveals that he is also a replica, created to take the place of the original Shuuya Sanada, a basketball star who became a shut-in after being bullied and injured. Calling himself Aki, he understands Naos situation in a way no one else can. Their connection becomes a lifeline for both of them, offering a sense of belonging that belongs only to them. This relationship strains the already fraught dynamic between Nao and Sunao, who grows jealous and resentful, treating Nao with increasing callousness as she sees her replica developing a life she feels she cannot have. The tension reaches a breaking point when a bully pushes Aki in front of a train, and Nao pushes him to safety, taking the impact herself. She is killed, but Sunao immediately summons a new copy, bringing Nao back.

This event changes their relationship. Sunao begins treating Nao with more kindness, allowing her to sleep in a bed and take baths for the first time, but the central conflict remains unresolved. As the story progresses, Nao must decide what she truly wants for herself, knowing that her existence depends entirely on the original who created her. She learns that other replicas exist, each created under different circumstances and with different understandings of their nature. The series explores whether Nao can claim her own love and identity as something genuinely hers, or whether she will always remain a shadow of someone elses life.