Mizusawa Nachi, a young man captivated by the beauty and kindness of a woman named Amagi Rina, experiences his first love. Despite their slight age difference, Nachi and Rina share a close bond akin to siblings, enjoying their days together. However, their relationship takes a turn during a shared bath when Nachi can no longer resist his desires and confesses his love to Rina, embracing her. Following the confession, their relationship evolves into a forbidden affair, filled with secret rendezvous after school, concealed from their parents. They promise to keep their relationship a secret. Their love deepens, and Nachi envisions marrying Rina in the future. However, their lives are disrupted by a deadly global virus outbreak, causing Rina's father's company, Tenjou Foods, to face financial troubles. As a result, Rina is forced to move to Tokyo. Before leaving, Rina reveals her desire to have a child with Nachi, hoping it would allow them to stay together. They consummate their love, but the next day, Rina disappears without a word, leaving Nachi heartbroken. Time passes, and Nachi successfully gains admission to Tokyo's prestigious university, seeking to find his lost love.