Shunsuke returns home for spring break only to discover that his three sisters are suffering from a rare condition called idiopathic estrus syndrome, which leaves them in a state of uncontrollable heat. Claiming that the only cure is the regular intake of male sperm, the sisters pressure Shunsuke into becoming their exclusive provider to avoid seeking strangers. As the situation escalates, even his childhood friend Haruka joins the fray, leaving Shunsuke overwhelmed by their insatiable demands.