The Sakurada Gate Incident recounts the story of Mito Domain Samurai coming off house arrest named Seki Tetsunosuke (played by Osawa Takao). He along with other prominent Mito Clansmen are tasked to assassinate the Tairo (Great Elder) Ii Naosuke (played by Ibu Masato) who in the Shogun Iesada's absence has signed treaties without Imperial Consent with the Americans in the Treaty of Amity & Commerce in order to head off Japan's most imminent threat to Japan that has sent the county into crisis. By entering into the unequal agreement with the Americans Ii Naosuke committed the crime of violating the Mikado's law (Emperor Komei). In response to such outrage the Mito Domain informed the Imperial Court and sought sanction. An Imperial Writ from the Mikado was given directly by the court to the Mito breaking the protocol which angered the Bakufu (Shogunal Government) in Edo. This resulted in the Ansei Purge - Ii Naosuke's reign of terror that resulted in over 100 arrests and eight executions of those who opposed Ii's actions which forces the hand of the Mito to take action. On a cold morning of March 1860 the assassination is carried out in a bloody sword battle. While Seki Tetsunosuke is merely tasked to record and confirm the Tairo's death, his battle for survival and that of the fruit of the Mito's actions that his men did not die in vain.
—Louis E. Rosas