In 1615, when Ōsaka Castle fell at the Summer Campaign of the Siege of Ōsaka, the formal wife of Toyotomi Hideyori and granddaughter of Ieyasu known as Senhime, was rescued from the castle. Thereafter, the treatment of Senhime led to a confrontation between Naomori and the bakufu whereupon Naomori devised a plot for her abduction.
Regarding the incident, under one theory, Naomori was requested directly by Ieyasu to help on the condition that he could marry Senhime, which was then disregarded. Under another theory, Ieyasu only said that he would give her to whomever saved her, and did not specifically request the assistance of Naomori. Moreover, with regard to whether Naomori rescued Senhime, there is a theory that Naomori did not actually perform the rescue, but that Senhime was protected by Horiuchi Ujihisa, a bushō with the Toyotomi. After Ujihisa brought Senhime to Naomori’s camp, Naomori sent her to Tokugawa Hidetada. Alternatively, there is a theory that, although Naomori rescued Senhime, he suffered burns in the process, and when she saw his appearance, she rejected him. According to some views, the story regarding her rejection of him is folklore.
Under another theory, the cause of the incident was not the rescue of Senhime, but Naomori’s loss of face. This occurred when it was suddenly decided that Senhime would wed Honda Tadatoki, the lord of the Himeji-Shinden domain, just as discussions had progressed via a Court noble to the stage of marriage with Naomori after Ieyasu had requested Naomori to set her future course now that she had become a widow.
Without regard to the reason, after the Summer Campaign of the Siege of Ōsaka, Naomori plotted to snatch Senhime, but these plans were discovered by the bakufu. The bakufu forces surrounded Naomori’s residence, and announced that if he committed seppuku, then a succession of the family would be permitted. There is a theory that the retainers refused to have their lord commit seppuku and were killed. Under another theory, the retainers were persuaded by the words of the bakufu leaders, and after Naomori fell asleep while intoxicated, he was slayed. Alternatively, based on the artifice of Tachibana Muneshige, Naomori killed himself after being reprimanded by Yagyū Muneyori. Moreover, based on the theory that Naomori was reprimanded, Muneyori received permission to use the family crest of the Sakazaki for the Yagyū clan.
These events were also detailed in the diary of Richard Cocks, the head of an English trading house who resided in Edo at the time.
At his defeat at the Battle of Sekigahara, Onodera Yoshimichi, the lord of Yokote Castle in Dewa Province, was dismissed from his duties and received protection from Naomori in Tsuwano. After the death of Naomori, Yoshimichi constructed a grave for Naomori as a debt of gratitude on the thirteenth anniversary of his demise.