The brother and sister are always fighting, to the point that their parents are annoyed with them. But in reality, all of these sibling fights are just an act to deceive their parents. In reality, their relationship has crossed the line between brother and sister and has become a relationship of lovers. So they get cozy when their parents aren't looking, and have sex as much as they want when their parents are out! The sense of guilt that they must never let their parents find out heightens each other's sensitivity, and they get sucked into a forbidden love affair!