"Ooku" begins in a world ravaged by a disease called Red Pox, which only affects men and leads to a shift in societal roles. Women now hold positions of power and perform heavy labor, while the men, weakened by the disease, are mostly confined and cared for.

In this first episode, a young man named Mizuno Seki Tomokazu from humble origins is allowed by the Chief Chamberlain Fujinami to train in a dojo. He is promoted to the rank of groom and assigned Sugishita as his assistant. However, it later turns out that Fujinami's kindness had an ulterior motive—he wanted to eliminate competition for his beloved Kashiwagi and took revenge on Mizuno for rising above his social station.

Although Mizuno faces prejudice due to his modest background, he demonstrates great practicality and knowledge of the outside world, which is largely unfamiliar to the palace nobility. His wisdom, kindness, and honesty make him popular among the lower ranks.

Even the Shogun, Yoshimune Kobayashi Sanae, takes notice of him. Mizuno's bravery is shown when he takes the blame for laughing during an audience over the Shogun's misstep, even though he wasn't responsible for it. When Mizuno is informed of his fate as a Secret Swan, he expresses concern for his family's well-being and accepts his destiny with dignity.

The episode also questions certain traditions. For example, Yoshimune is seen engaging in intimate activities with various men, even though the Shogun is expected to be inexperienced in such matters. This leads to a pleasant twist where Mizuno's death is faked so he can return to his true love, O-Nobu.

Throughout the episode, Yoshimune and her servant Hisamichi demonstrate intelligence and tact by thwarting the political schemes and manipulations of the palace. They use a tradition of the garden festival to dismiss unwanted men from service and expose corruption in the palace administration.

The episode is also visually appealing, showcasing beautiful kimonos that reflect the personalities of the characters and the significance of the events. Particularly noteworthy are Yoshimune's formal kimono and Mizuno's simple black outfit.