Naoto Watari has had his share of struggles: his parents died two years ago, and now he and his sister live at his aunt's house. There, his desire to protect his sister earned him notoriety as “the guy with the sister complex”-which never mattered to him, as his sister always came first. But when Satsuki, a childhood friend-turned-enemy(?), becomes his school mate, Naoto's ordinary life is thrown into chaos … and Satsuki may just be the beginning.