Miharu Mashiki is a first-year high school student defined by a critical heart condition that leaves her medically vulnerable. She carries the tragic history of her identical twin sister, Misora, who succumbed to the same heart disease during their childhood.

Her life intersects significantly with Haruki Mishima, a classmate and swim club member. Haruki met Misora in kindergarten when she read him a fairy tale and gave him the book. Years later, he discovered that girl was Miharu's deceased twin. Despite this connection, Haruki developed romantic feelings for Miharu. Their relationship rekindles after a reunion at a convenience store near their high school.

Miharu reciprocates Haruki's romantic feelings. Their initial interactions are marked by mutual awkwardness and hesitance, reflecting the emotional complexity tied to their shared history with Misora. Miharu demonstrates kindness and supportiveness, notably collaborating with Haruki to arrange a Christmas gathering aimed at reconciling their estranged friends, Towa Honda and Mami Mihashi.

As the new year progresses, Miharu's health deteriorates markedly, causing frequent school absences due to hospital visits and consultations. Haruki visits her daily during her hospitalization, showing deepening commitment. Her condition necessitates a major heart surgery, requiring transfer to a specialized hospital. This impending surgery causes Miharu profound anxiety, intensified by the separation from her school life and friends.

The progression of her health crisis introduces intense emotional stakes into her relationship with Haruki. Their evolving dynamic unfolds against the backdrop of her illness, including moments where they prioritize supporting friends over their own uncertainties. Her condition and the constant awareness of her sister's fate underscore themes of fragility and the urgency of emotional connection.