The members of the Umino household are all rather similar: they are energetic and incredibly dynamic—except for Nagi, a serious and introverted young man whose sole focus is on academics. He is the complete opposite of someone like his younger sister Sachi, who is a reflection of her extroverted parents. Thus, it should come as no surprise that Nagi was swapped with another baby at birth. On the way to meet his true parents, Nagi encounters Erika Amano, a young social media star. While spending the day with her, he learns that Erika's parents forced a marriage onto her. Later that day, Nagi comes to learn that Erika's fiancé is actually none other than himself, in order to officially be part of each other's family. With the goal to build the pair's relationship, Nagi's biological father arranges for the fiancés to start living together! However, no matter what tricks their parents pull, Nagi refuses to marry Erika. That is because he already has sights set on someone else: the beautiful and intelligent Hiro Segawa.