In the summer of the second year of junior high school, Yume Ayai confessed to her classmate, Mizuto Irito , and the two began dating. At first, the two spend time as a couple, but due to a slight misunderstanding, they become estranged from each other and break up after graduating from junior high school.
However, two weeks later, Mizuto's father and Yui's mother, who were both single parents, remarried, and Mizuto and Yui became siblings with the same surname of Irito, and in the same house. Started their new life. Mizuto and Yui are concerned about their parents' newlywed life and decide to hide that they were a couple and treat them as just siblings in law, even though they can't get rid of their illness when they were dating. ..
Then, the two who went on to the same high school and had the same class, Kogure Kawanami who tried to develop Mizuto and Yui into a romantic relationship, Minami Akatsuki who has a one-sided and excessive affection for Yui , Mizu Meet three classmates, Isana Higashito , who falls in love with Doo for the first time, and become friends. After that, it turned out that Kogure and Akatsuki were dating when they were in junior high school, and Isana confessed to Mizuto, and the relationship between the five became complicated with a mixture of romantic elements. To go.
Meanwhile, the Irito family returned to the hometown of Mizuto's father with four family members, and there he decided to learn about Mizuto's past and his thoughts that he could not know even when he was dating. Become. Recognizing that his feelings for Mizuto were not exhausted, he decided to break away from his past love relationship with Mizuto and aim for a new relationship with Mizuto as Yui Irito.