The one who hurriedly ascended the castle upon hearing news of Shogun Tsunayoshi's illness was Saotome Mondonosuke. However, instead of an illness, he was prompted to consider a marriage proposal with Yaehime, a daughter of the Date clan. Yet, this was merely a pretext, as he was requested to investigate the truth behind the rumors of rebellion within the Date clan. Mondonosuke, who took on this task to stave off boredom, accompanied by his retainer Kinai, departed for Mutsu, following the procession of Yaehime returning to her home province. However, remnants of the Toyotomi faction, already plotting the overthrow of the shogunate, had gathered under Kii Koya Bunzaemon…

将軍綱吉（長門裕之）病気の報せに急ぎ登城した早乙女主水之介（北大路欣也）だったが、病気どころか、伊達家の息女・八重姫（多岐川裕美）との縁談を勧められる始末。だが、それも口実で、伊達藩謀反の噂の真偽を探ってきてほしいとのことだった。退屈の虫封じにと引き受けた主水之介は、用人の喜内（堺正章）を連れ、国許へ帰参する八重姫の行列を追って陸奥へと旅立った。だが、すでに幕府転覆を狙う豊臣方の残党たちが、紀伊国屋文左衛門（名古屋章）のもとに集結していた……。