On June 8, 1651, Tokugawa Iemitsu (徳川家光, Tokugawa Iemitsu) — the third and most formidable of the Tokugawa shoguns — died of a stroke at the age of 46. He left behind a ten-year-old heir named Ietsuna (家綱, Ietsuna), a shogunate full of nervous officials, and a social catastrophe decades in the making. His 28-year reign had been built on the complete subordination of every competing power to the Tokugawa state — and one instrument of that subordination had become a time bomb.

Iemitsu's policy of han (藩, han) confiscation was administratively elegant and humanly ruinous. Any daimyo (大名, daimyo) who died without a pre-approved heir had his domain absorbed by the shogunate. Daimyo who died unexpectedly, without time to seek official permission to name a successor, lost everything — and so did every samurai in their service. Under Iemitsu's reign, roughly 150,000 samurai had been stripped of their positions and income. Some estimates put the total ronin (浪人, ronin) population of Japan in 1651 at nearly half a million — men trained for combat, living in poverty, with no honorable place in a society that had decided peace was permanent.

Into this tension stepped Yui Shosetsu (由比正雪, Yui Shosetsu). Born of humble origins in Sunpu (駿府, Sunpu) around 1605, Shosetsu was a man of uncommon brilliance. He had trained in military strategy and swordsmanship, eventually opening a prestigious academy in Kanda (神田, Kanda), Edo, that drew hundreds of students — men drawn disproportionately from the ranks of the angry and the dispossessed. He ran an armorer's shop alongside his school. He was respected enough to be counted among the Three Great Ronin of his era. He was also, by approximately 1645, in the process of planning a revolution.

His co-conspirator was Marubashi Chuya (丸橋忠弥, Marubashi Chuya), a swordsmanship and halberd instructor with a personal grievance: his father had been killed in the Osaka Campaign (大坂の役, Osaka no Eki) of 1615, in which the Tokugawa had extinguished the last rival to their power. Marubashi would remain in Edo. Shosetsu would travel to Sunpu. At the appointed moment, gunpowder barrels hidden across Edo's densely packed wooden neighborhoods would be ignited simultaneously, setting the city ablaze. While officials fought fires and the castle compound fell into chaos, Marubashi's ronin would storm Edo Castle (江戸城, Edo-jo) and kill the shogunate's senior leadership. Shosetsu would seize the treasury at Sunpu. The shogunate, headless and its capital in flames, would be forced to reverse the han-confiscation policies — to restore the tens of thousands of samurai Iemitsu's machine had rendered masterless.

The plan was not fanciful. Edo was an overwhelmingly wooden city. Just two years after the uprising's failure, in 1657, the Great Meireki Fire (明暦の大火, Meireki no Taika) would consume most of the city and kill an estimated 100,000 people — and no one had to plan that. A deliberate, coordinated ignition could have caused genuine catastrophe. The conspirators had men, weapons, and a cause that resonated across an entire aggrieved social class.

What they did not account for was Marubashi's body. In the weeks before the attack, he fell dangerously ill. In the delirium of his fever, he talked — and his words reached government informers. Senior councilor Matsudaira Nobutsuna (松平信綱, Matsudaira Nobutsuna) had Marubashi arrested before he could act. Shosetsu, learning of the arrest while in Sunpu, found himself surrounded. He committed seppuku (切腹, seppuku) on September 10, 1651, rather than surrender. Marubashi was crucified, his body displayed publicly alongside 33 of his fellow conspirators. Their family members were executed as well. Six years of meticulous conspiracy had been undone by a man's fever.

The aftermath was, paradoxically, a partial victory for the conspirators' cause. Within days of the executions, the shogunate announced reforms to inheritance law: daimyo would now be allowed to designate heirs even on their deathbeds, without advance approval. The policy that had created hundreds of thousands of ronin was quietly reversed. The men who had plotted to burn Edo failed militarily and won politically. The Keian Uprising (慶安の変, Keian no Hen) would prove to be the last serious attempt to overthrow the Tokugawa shogunate from within the warrior class for over two hundred years.