When Kōta's father remarries, his world is turned upside down by the arrival of his new stepsister, Aika. From their first encounter, it is clear she is everything he is not. Aika presents a carefully maintained dual personality to the world. By day, she plays the role of the perfect angel for her parents, but behind closed doors, she reveals her true self as a cheeky, foul-mouthed gyaru whose bratty and uncontrollable behavior makes Kōta’s life a living hell. Their clashing personalities spark constant arguments the moment their eyes meet, locking them in a seemingly endless battle of wills as the world’s worst siblings.

The story unfolds primarily within their shared home and school, where the two are forced to navigate their new, complicated family dynamic. Despite an incredibly rocky start, a subtle shift begins as they coexist. Beneath Aika’s wild and provocative exterior, a flicker of genuine interest starts to grow for her new stepbrother. Aika gradually warms up to Kōta’s caring and patient nature, and their awful relationship begins to change in a dramatic and intimate direction. What begins as a tense cohabitation filled with insults and provocations slowly blurs the lines between family and romance, transforming their relationship from hostile to sexual.