ご存知！旗本退屈男Ⅳ 紀州徳川家滅亡か？！

A marriage proposal from the Kishu family is presented to Kikuji, but her brother, Sawatari Shusui, declines it. Around the same time, the lord of the Kishu family dies under suspicious circumstances. Determined to resolve the turmoil within the Kishu domain, Shusui embarks on a journey. The proposed suitor for Kikuji is Shin'nosuke, a young samurai who recently saved Shusui from an attack but left without receiving thanks. Shin'nosuke, heir to the Kishu domain, has headed back to Kishu. However, Iwakura Guntayu, the Edo elder of the Kishu domain, is plotting to install his concubine's child as the new lord and seeks Shin'nosuke's life. Accompanied by Kikuji and his servant, Sasao Yoshinari, Shusui hastens to Kishu to confront the brewing crisis.