Reina, the disciplined Student Council President, struggles to manage her non-blood-related younger brother, Shouta, whose obsessive "sister complex" leads to constant harassment. Despite her efforts to maintain order while their parents are abroad, Reina eventually succumbs to Shouta's manipulations and her own hidden desires, agreeing to become his devoted "pet." Their forbidden relationship reaches a breaking point when Aina, the younger sister, unexpectedly returns home to find them together.