At the age of seven, a girl named Sunao Aikawa makes a spontaneous wish to avoid a difficult social situation, and a perfect copy of herself materializes to help. This replica, whom Sunao refers to simply as Second, is a supernatural entity bound to serve her original. Sunao can summon and dismiss her at will, and for ten years, this arrangement continues without question. Now a teenager, Sunao lives a reclusive life, sending her replica to school on days she feels sick, has a test, or simply cannot be bothered to attend. The replica dutifully performs the exhausting work of living Sunao’s life for her, from attending classes to navigating friendships, believing her entire purpose is to be a convenient substitute.

As the replica begins to experience the world firsthand, her personality starts to diverge dramatically from the sullen and cynical Sunao. She discovers a love for literature, forms genuine bonds with classmates, and eventually names herself Nao, a small but significant act of self-definition. The central conflict ignites when a boy named Shuya Sanada returns to school after recovering from a basketball injury. Sanada is the first person to notice that the girl he is talking to is not the same Sunao who ignores him on other days. Fascinated by Nao, he agrees to a secret system where he will only interact with her when she wears her hair in a half-up style.

Nao finds herself falling in love for the first time, a feeling she is certain belongs to her alone, despite her borrowed name and body. However, her existence is precarious. She has no legal status, no possessions, and no guarantee of a future. Sunao, who believes her replica has no real feelings, grows increasingly jealous and controlling, punishing Nao by dismissing her into non-existence for days at a time. The story unfolds as Nao navigates the bittersweet joy of her first romance while living under the constant threat of being erased by the very person she was created to serve. The narrative explores her struggle to carve out an identity, the family members who begin to suspect the truth, and the question of whether a life lived in the shadows can ever truly become her own.