ご存知！旗本退屈男Ⅲ 疑惑渦巻く大奥！

One night, Sawatari Shusui saves a geisha named Osode from a ninja attack. Osode's mother is said to be carrying the child of the shogun, Tsunayoshi, and has hidden herself on Hachijo Island. Shusui heads to Hachijo to verify the truth. Meanwhile, Shusui had heard from Tsunayoshi that he was planning to appoint the Kofu Chancellor, Tokugawa Tsunatoyo, as the next shogun, but Sumi's father, Makino Bizen no Kami, whose daughter was pregnant with Tsunayoshi's child, was planning to have his daughter's child ascend to the shogunate and seize real power. Amidst all this, Shusui, along with Osode and his servant, Sasao Yoshinari , sets foot on Hachijo Island and meets Tsunayoshi's son, Kotaro, who was born to Osode's mother. But then a group of ninjas attacks.