After being swapped at birth, Nagi Umino is currently a second-year student at a prestigious high school. One day, he runs into Erika Amano, a spoiled rich high school girl who says she's on the way to meet the fiancé her parents arranged for her, and ends up pretending to be her boyfriend. But the two of them soon learn that they'd been swapped with each other as babies, and their parents decided to engage them to each other... Nagi and Erika have a new addition to their household: their younger sister Sachi! Nagi and Sachi have a big fight when he tries to send her back to their parents, but Sachi is actually just unable to say how she wants to spend more time with her big brother. Throw in an amusement park date with Nagi's beloved Hiro and some rapid bonding with Erika and this four-way love knot quickly grows more and more complicated!