The protagonist, Misato Fukui (name changeable), lives a lively and harmonious life surrounded by his three unique sisters. One day, his father remarries, and he gains an older stepsister, Yuuka, who’s just one year his senior. Yuuka is shy and struggles to fit in with the family, and Misato can’t just leave her be. But his three sisters aren’t exactly calm about the situation. The eldest sister clings to him, showering him with affection as if to show Yuuka how close they are. The middle sister pretends not to care, but behind the scenes, she aggressively seeks out intimate moments. The youngest sister, torn by her growing feelings for her brother, can’t stop herself from acting on them. At home and at school, the sisters try to tempt the guy. Will Misato’s school life and the peace of the Fukui household hold… or will it all come crashing down!?