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! On the way back from their heart-pounding beach trip, Nagi and Hiro miss their train and end up staying the night in an inn... Later, Nagi is in for a shock when Hiro runs away from home to join the other three in their house! The midsummer love rectangle grows hotter and hotter!