16-year-old super-studier Nagi Umino, second-year student at the Meguro River Academy high school, was switched at birth. On his way to a dinner to meet his birth parents, he accidentally meets the brash, outspoken, Erika Amano, who is determined to make Nagi her fake boyfriend as she never wants to actually marry. But once Nagi makes it to dinner, he finds his parents have decided to resolve the hospital switch by conveniently having him marry the daughter his birth parents raised...who turns out to be none other than Erika herself!
Nagi's crush Hiro has found out about his engagement to Erika! Hiro can't hide her surprise, but after she tells Erika about her own engagement, they form the "engagement alliance." Meanwhile, Sachi is starting to feel suspicious of Hiro... And when the gang heads off to a study camp, their love square heads past the point of no return!