A high school boy named Nao is strikingly handsome and popular, but his heart belongs to one person: his older stepbrother Yuri. Nao had planned to confess his feelings after entering high school, but his parents' remarriage made Yuri his official brother, and he lost the chance to speak. When Nao discovers that Yuri has someone he likes, he resolves to bury his romantic feelings and commit to being a proper family member instead. Despite this, the two grow closer in secret, and their mutual attraction eventually surfaces. They begin a hidden relationship, navigating the tension between their new bond as brothers and their desire for each other. The story follows their forbidden, comedic romance as they steal moments together at home, overcome milestones such as their first time, and face the challenge of keeping their love concealed from their family. The manga builds toward a Christmas vacation where Nai tries to propose, and the series concludes with the pair solidifying their future together. Set in a contemporary Japanese household, the narrative focuses on the private, emotionally charged world of two step-siblings discovering that brotherly affection and romantic love cannot be so easily separated.