Except Nagi, who is reserved and introverted, and solely focused on his studies, the rest of the Umino household is full of energy and dynamism. His younger sister, Sachi, is a spitting image of her outgoing parents. Nagi realizes that he was probably switched with another baby at birth, which explains his stark contrast in personality. While on his way to meet his biological parents, Nagi encounters Erika Amano, a popular social media figure. Their day together allows Erika to open up to Nagi about her parents' forcing her into an arranged marriage. Nagi is taken aback when he finds out that he himself is Erika's intended fiancé, and their parents arrange for them to live together to form a strong bond. Nagi, however, is already in love with the stunning and brilliant Hiro Segawa, and despite their parents' plans, refuses to marry Erika.