Ooku is set in an alternate version of Japan in which a disease has drastically decreased the male population, turning the nation into a matriarchal society in which young men are treated as a precious commodity never to be risked in sports or war. Hiroyuki Mizuno (Kazunari Ninomiya) is a commoner from a struggling samurai family who isn’t allowed to marry the woman he loves because he’s too poor to afford that right. To earn money for his family, he decides to enter Ooku (the palace), where young men like him are simply added to a harem as a status symbol for the wealthy and powerful. Mizuno quickly establishes himself as someone not be be trifled with, setting himself apart from the other men. However, when the unmarried Yoshimune (Kou Shibasaki) takes over as Shogun and chooses Mizuno as her first consort, a long-standing Ooku tradition threatens to cut their relationship very short.