Lala and Kiki, the twin daughters and heirs of the Hoshino conglomerate, were living a comfortable life in a luxurious mansion near central Tokyo, attended by maid Maami and butler Tanaka despite still being in high school. One day, upon returning home from school, they received a letter from their mother Tsumiki—the true "Lady Sovereign" of the Hoshino family—who had been undergoing long-term recuperation at a sanatorium. The letter contained a shocking revelation: when the twins were young, following the Hoshino family tradition, they had been placed in a master-servant relationship known as "Lady Sovereign and Consort," but their father Minoru, who betrayed Tsumiki, had his face and body burned with sulfuric acid and was still being kept imprisoned as a living slave in the mansion's basement. Having been told since childhood that their father died in a traffic accident, the girls were stunned by this truth—but even more shocking was the fact that Minoru had a secret illegitimate child older than them: Arata, who was now the homeroom teacher at the girls' school. Tsumiki ordered the sisters to establish themselves as full-fledged "Lady Sovereigns" by deliberately degrading their half-brother Arata into a subhuman beast. Having been raised under Tsumiki since childhood, trained as sadists using their father and butler Tanaka's bodies as teaching tools, Lala and Kiki readily accepted the command. With help from their classmates Mio, Rui, Mai, and Mirei, they lured Arata into a trap, subjected him to brutal sadistic torture, broke his mind and body, then successfully carried out a plan to lock him in the school's equipment room after class for several days and force him to fall into a public toilet. Yet unsatisfied, Lala and Kiki escalated further—kidnapping Arata, forcibly bringing him to their home under the guise of a house visit, imprisoning him in the mansion's storage room, and, with their classmates' continued assistance, devising even bolder schemes, including exclusive oral worship sessions performed on them and ultimately transforming him into a human golden toilet.