Six years of planning. A network of 浪人 (rōnin) spread across Edo and Sunpu. A plot to detonate the Tokugawa gunpowder arsenal, burn the city in the chaos, assassinate senior officials, and seize 江戸城 (Edo-jō, Edo Castle) from a ten-year-old shogun. And then one man got sick.

It was the summer of 慶安四年 (Keian yonen) — the fourth year of the Keian era, 1651 — and the timing had been chosen with precision. Tokugawa Iemitsu 徳川家光 (Tokugawa Iemitsu) had died in June, leaving behind Tokugawa Ietsuna 徳川家綱 (Tokugawa Ietsuna), age ten, as Japan's fourth shogun. The Tokugawa regime had never been more exposed. An untested child on the throne, powerful vassals watching, a country full of displaced warriors — the conspirators believed this was the moment.

由井正雪 (Yui Shōsetsu, Yui Shōsetsu) was not a villain out of a shadow play. He ran a respected school of military strategy in Kanda, Edo, and was considered one of the era's great martial scholars — named one of the "Three Great 浪人 (rōnin)" alongside Kumazawa Banzan and Yamaga Sokō. His conspiracy grew inside a legitimate institution, in plain sight, over six years. His co-conspirator, 丸橋忠弥 (Marubashi Chūya, Marubashi Chūya), was a martial arts instructor with genuine students. These were not desperate brigands. They were educated, skilled men who had been discarded by a system designed to produce exactly their grievance.

The Tokugawa succession policies — seizing domains on technical pretexts, denying inheritance to adopted heirs — had created tens of thousands of 浪人 (rōnin) in the preceding decades. Men trained for loyalty and war, suddenly without master or income. Yui Shōsetsu intended to weaponize that social kindling.

The plan called for simultaneous attacks on Edo, Sunpu (modern Shizuoka), and the shogunal shrine at Kunōzan. In Edo, the gunpowder arsenal would be detonated — essentially blowing the city apart from within — while the ensuing chaos was used to eliminate senior Tokugawa officials and storm the castle. The shogunate would not fall in battle. It would fall in fire and confusion.

Then 丸橋忠弥 (Marubashi Chūya) collapsed with a high fever. Delirious, burning, he talked in his sleep. Informants heard enough to bring it to the authorities. Senior councillor 松平信綱 (Matsudaira Nobutsuna, Matsudaira Nobutsuna) moved quickly. 丸橋忠弥 (Marubashi Chūya) was arrested along with thirty-three co-conspirators. Yui Shōsetsu, surrounded by shogunate police in Sunpu, committed 切腹 (seppuku). The fuses were never lit.

丸橋忠弥 (Marubashi Chūya) was executed on September 24, 1651 — by 磔 (haritsuke, crucifixion), the method reserved for traitors, chosen specifically to prevent any legacy of martyrdom. Male family members were also executed.

The shogunate responded to the uprising by doing something surprising: it quietly relaxed the succession policies that had created so many 浪人 (rōnin) in the first place. The plot failed completely — and partly succeeded anyway. Japan would not face another serious internal challenge to Tokugawa rule for over two hundred years.