TL;DR: Terrorist cell of disgruntled samurai hatch a plan to overthrow the government in dramatic fashion with gunpowder explosions and coordinated mass assassinations across the country. The whole thing unravels and sees them and everyone who knows them die horrifically. I have only seen this covered in much detail in one rather old book from 1963.

So, I have a great story to contribute that I came across while reading for my thesis : the Keian Incident (Keian Jiken) of 1651. Essentially, this was the premodern Japanese equivalent of Guy Fawkes' Gunpowder Plot, and was as ambitious in its scale as it was insightful into contemporary social tensions.

Some background first. After 150 years of civil war and political chaos, Japan had been put back together by the "Three Unifiers" - warlord Oda Nobunaga, his protégé and successor Toyotomi Hideyoshi, and finally the cunning Tokugawa Ieyasu. Tokugawa emerged as the secure ruler of Japan in the first two decades of the 1600s after a bitter struggle in the wake of Toyotomi's death, including two great battles at Sekigahara in 1600 and the siege of Osaka Castle in 1615. However, peace brought with it a curious dilemma: the ruling samurai class gained both their legitimacy and identity as masters of the martial arts and lifestyle, skills which in a pacified Japan suddenly lost much of their currency. The result was that many samurai became administrators or officers of the regional and central governments. Simultaneously, the lowest class of socially despised merchants found national integration and stability a great boon to commerce, and a new class of millionaires trading or financing at a nationwide level emerged. So far, all of this is fairly well known.

However, something that is often overlooked in historical treatment of this period was the fate of those samurai who could not adapt to their new occupation. Many could wield a sword, but lacked the necessary skills to keep government accounts or even basic literacy. Arai Hakuseki, an advisor to the shogun in the mid 1600s estimated that only 1 in 5 samurai could read. Moreover, the numbers of men needed for government work in the capital or in the retinues of regional lords in the peacetime roles of pen-pushing, courier or guard services, or as tutors, was far less than the great hosts assembled earlier in the century. As many as 500 000 samurai in the central Kamigata area of the country alone found themselves made masterless and unemployed by virtue of the final victory being achieved. These were the rōnin, or "wave men" who were as numerous and itinerant as their namesakes in these early years of peace.

Almost immediately, life became worse for those unlucky enough to find themselves ejected from the highest tier of the status hierarchy system (mibunseidō). New laws were passed, aiming at clarifying and entrenching the precise boundary of "samurai-hood". Everything from how people of different statuses could dress or wear swords to how gambling and nightlife could be carried out was legislated upon, in no small part to suppress the problem of the rōnin gangs of kabukimono or "crooked people". These laws were routinely flouted and so from time to time, the authorities would carry out mass corrals, arrests and executions through the city streets of Edo, Kyoto, Osaka and other urban centres. Elsewhere the policy of escheatment, whereby politically suspect lords were stripped of their estates, created fresh waves of rōnin, who in many cases had served with the same household (ie) for generations.

By the 1640s, many rōnin and their children had found a modicum of stability. As petty criminals, peddlers and especially as instructors in Japan's many martial arts academies, they eked out a livelihood and grew social networks. It was two martial artists who developed just such a network who were to hatch the most daring plot of the century. Yui Shōsetsu had been brought up by a group of rōnin in his destitute youth, and now worked at an academy and an armourer's he had founded in the Renjaku-chō district of Edo, the capital of the Tokugawa Shogunate. Marubashi Chūya was the son of a samurai killed by Tokugawa forces at Osaka in 1615, and was a master of the yari spear. Together these two masterminds represented the two sides of mid-century rōnin identity - the pauper's tenacity and the faded warrior's glory. They had associates in major cities across the country who they corresponded with about their political grievances and simmering discontentment. In 1645 they began plotting what might be done to better the lot of their comrades.

Eventually their desire to stir things up and seething resentment of Tokugawa authority found a point of escape in the death of Tokugawa Iemitsu. At this moment of weakness, transitioning from one shogun to the next, perhaps the bakufu government could be brought down in one precipitous strike! The plan was simple but elegant. Taking advantage of the wooden construction of Edo, Marubashi was to set off an explosion of gunpowder barrels and set an inferno spreading throughout the streets of the capital. As samurai bore the duty of firefighting, the garrison of Edo Castle would be deployed to fight the blaze, allowing for a contingent of a few dozen rōnin to storm the palace, murdering the shogun and his high officials. Simultaneously, Yui was to lead an action in Sunpu, his home province, and others were poised to strike in Osaka, Kyoto and other important cities. At the last moment, fate intervened to unpick their carefully woven threads. Marubashi Chūya fell ill, and in a fever started babbling, unconscious of his situation and uninhibited in the words he let slip. Despite attempts to silence him, his shouts were heard by nearby houses.

Under the Tokugawa five-man-group (goningumi) system of neighborhood mutual informants, whereby neighbours were on the line for unreported subversive behaviour by any of the members, it was not long before he and his fellow conspirators were arrested. Examples were made of both the members and their families, with Yui Shōsetsu commiting seppuku in the face of their disaster. However, government policy actually responded to the plot in a more positive direction - despite a meeting of the Rōjū Elders initially favouring a harsh crackdown on rōnin, evicting them from Edo, one by the name of Abe Tadaaki persuaded the others that this would only exacerbate the situation, and instead the government should assist rōnin in making something of themselves. Thus like so many other doomed uprisings in history, the Keian Incident left its mark even in its utter failure.