Yuiko Hizuki, a scion of the Hizuki clan, is wed to Masakazu—the architect of a fatal car accident that killed Suzune, Naoto Rindou’s fiancée, and fractured the Rindou family’s ancient pact. This covenant compels a Hizuki woman to unite with a Rindou heir every three generations through the Hinotsukisai ceremony, a ritual vital to their legacy. Yuiko strives to honor this obligation by conceiving Naoto’s child, but infertility bars her path. Undeterred, she pivots to her sisters—Shino, Ayumi, and Mitsuna—as substitutes, propelled by the Rindou’s dwindling timeline and the crushing expectations of duty. Her resolve, shaped by her husband’s transgressions and her clan’s intertwined fate, cements her as the linchpin in a struggle to balance tradition’s demands against the fragility of human agency.