In the early summer of 1651, the Tokugawa shogunate (徳川幕府, Tokugawa bakufu) faced its most dangerous moment since Ieyasu (家康, Ieyasu) had won the Battle of Sekigahara (関ヶ原, Sekigahara) half a century before. The third shogun, Tokugawa Iemitsu (徳川家光), had ruled with an iron authority that kept every daimyo (大名) and every rōnin (浪人) in line. On June 8, 1651, he died at the age of 46, leaving behind a ten-year-old heir named Ietsuna (家綱, Ietsuna) and a government propped up by regents. For two men who had spent six years in secret preparation, the moment to strike had come.

Yui Shōsetsu (由比正雪) was 46 years old and one of the most respected military scholars in Edo (江戸). He operated a school of martial strategy in the Renjaku-chō neighborhood of Kanda, where he taught samurai, daimyo retainers, and anyone who could pay. His co-conspirator, Marubashi Chūya (丸橋忠弥), ran a parallel school of martial arts nearby. Between them, beginning around 1645, they began quietly recruiting from a population that had every reason to hate the shogunate.

The source of that hatred was a policy decision made in the name of stability. The Tokugawa shogunate had used attainder — the forced seizure of a daimyo's domain — as a tool for eliminating political rivals and consolidating control. Between 1600 and 1650, roughly sixty-one domains had been confiscated. Each confiscation meant hundreds or thousands of samurai suddenly had no lord, no stipend, and no future. By the 1650s, contemporaries estimated Japan's rōnin population at 400,000 to 500,000 — roughly one-fifth of the entire samurai class. Trained fighters with nothing to lose.

Yui and Marubashi's plan matched the scale of their grievance. They would wait for a moment of weakness in the shogunate — a succession crisis, a death in the ruling family — and then strike simultaneously in two cities. Conspirators would set Edo ablaze, using the chaos to assassinate shogunal officials and seize Edo Castle. At the same moment, Yui would lead a strike in Sunpu (駿府) to capture the Tokugawa clan's military treasury at the shrine at Kunōzan (久能山). A double blow in two cities, timed to the moment when the government was most distracted.

Iemitsu's death was that moment. But the conspiracy unraveled from the most mundane cause imaginable. Marubashi Chūya fell gravely ill with a fever. In his delirium, he raved about the plan — fires, castles, the treasury, revolution — not knowing who was in the room. Government informants heard enough. Marubashi was arrested before a single building was torched. Yui Shōsetsu, learning that the conspiracy had been exposed, found himself surrounded by authorities in Sunpu and committed seppuku (切腹) on September 10, 1651. Marubashi and thirty-three fellow conspirators — along with their male family members — were executed by crucifixion on September 24, 1651. Crucifixion was a punishment reserved for the worst common criminals; applying it to samurai was a calculated public statement that these men had forfeited every claim to warrior dignity.

What Yui left behind was almost as remarkable as the plot itself. His suicide note claimed that his goal had never been to overthrow the Tokugawa — only to force a reform of the shogunate's treatment of rōnin. Historians still debate whether that was sincere, a final piece of calculated rhetoric, or an attempt to protect his surviving associates. What is not debated is that the grievance he named was real.

Shogunal Elder Abe Tadaaki (阿部忠秋) made the argument that now had to be made: mass punishment of rōnin would only deepen the crisis. The root cause — the relentless production of masterless men through domain confiscation — had to stop. The shogunal council agreed. The law of escheatment was reformed. Daimyo were permitted to designate heirs by deathbed adoption, a practice previously restricted, which dramatically reduced the rate of domain seizure. The flow of dispossessed samurai slowed. Under Ietsuna (徳川家綱), the fourth shogun, daimyo reassignments fell sharply. A conspiracy that never lit a single fire had, through its failure, reformed the policy that made it necessary.

Yui Shōsetsu is remembered today as one of the "Three Great Rōnin" of the Edo period — a category that acknowledges the period's strangest contradiction: that some of the most formidable intellectual and political challengers to the shogunate's order were the very men the shogunate's policies had cast adrift.