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 is feeling anguished after learning about Erika's older brother, so he goes to Erika's hotel mogul father to find out the truth. But his plan ends up backfiring as the situation grows even more complicated...though it's nothing a bike date with Hiro and a beach trip with all three girls can't fix! Time for Nagi's ultimate summer to begin!