The Umino family is full of lively and energetic personalities — except for Nagi, a quiet and studious boy whose life revolves around academics. He’s the polar opposite of his bubbly younger sister Sachi, who takes after their equally outgoing parents. It’s no wonder, then, that Nagi discovers he was accidentally swapped with another baby at birth. On the way to meet his biological parents, Nagi crosses paths with Erika Amano, a flashy social media celebrity. Spending the day with her, he learns that Erika’s parents have arranged a marriage for her — and to his shock, the fiancé turns out to be him! In an attempt to bring the two closer, Nagi’s real father even arranges for them to live together. But despite their parents’ schemes, Nagi refuses to go along with the marriage plan. After all, his heart already belongs to someone else: the smart and beautiful Hiro Segawa.