Edo-period Japan stands strong and proud, decades after its imminent doom was prophesied. The new shogun, Yoshimune "Nobu" Tokugawa, encounters an array of traditions that, to her modern judgment, make little sense. An epidemic exclusively rages among men, resulting in men making up only one-fifth of the population. Still, three thousand young, agile, and beautiful men serve in the Oooku—the shogun's Inner Chambers. To unearth the origins of the existing hierarchy where women head their families and govern the nation, Nobu reviews the chronicles of the odd disease. Customs and traditions have changed to accommodate for the plague; these new laws dictate the current culture and reflect the cruel story of the first female shogun, Iemitsu Tokugawa. Most notably, these customs seem to lead back to the moment a young monk, Arikoto Madenokoji, must abandon his way of life and devote his remaining days to the shogun and her Oooku. [Written by MAL Rewrite]

Oooku was released on Blu-ray as a two-disc set by Netflix on July 20, 2023. The anime adapts the first 15 chapters of the manga.