A group of assassins come together for a suicide mission to kill an evil lord.
In 1844, the peace of Feudal Japan is threatened by cruel Lord Naritsugu Matsudaira, who is politically rising and getting closer to his half-brother, the shogun. After the harakiri of the Namiya clan leader, samurai Shinzaemon Shimada is summoned by the shogun's advisor Sir Doi of the Akashi Clan to listen to the tragedy of Makino Uneme, whose son and daughter-in-law have been murdered by Naritsugu. Then Sir Doi shows a woman with arms, legs and tongue severed by Naritsugu and she writes with her forearm a request to Shinza to slaughter Naritsugu and his samurai. Shinza promises to kill Naritsugu and he gathers eleven other samurais and plots a plan to attack Naritsugu in his trip back to the Akashi land. But the cunning samurai Hanbei Kitou that is responsible for the security of his master foresees Shinza's intent. Shinza decides to go with his samurai through the mountain, where they find the hunter Koyata that guides them off the mountain and joins the group. Now the thirteen men prepare an ambush to Naritsugu and his army of two hundred samurai in a suicide mission to stop evil.—Claudio Carvalho, Rio de Janeiro, Brazil
When his tyranny runs rampant, samurai Shinzaemon Shimada (Yakusho) is charged with ensuring Naritsugu's bloody reign ends. With the help of twelve samurai, Shinzaemon must set a trap for the Lord and his 200 men that will prevent him from ascending the throne.
1840s Japan is the final decades of the Tokugawa shogunate. Shogun was the title of the military rulers of Japan during most of the period spanning from 1185 to 1868.The sadistic younger stepbrother of the current Shogun, Lord Matsudaira Naritsugu of Akashi, sexually assaults and kills at will. Sir Doi Toshitsura (Mikijiro Hira), the Shogun's Justice, realizes the situation will become more dangerous after Naritsugu ascends to a higher political position. Sir Doi Toshitsura knows that this ascendance will cause civil war between the Shogun and the many feudal lords Naritsugu has offended.
On March 5, 1844, the Akashi clan house elder Zusho Mimiya publicly commits Harakiri (Harakiri denotes the only way in ancient Japanese way in which slaves demonstrate rejection of their masters. Harakiri means that a slave would rather die by slashing their body rather than continue slaving) as a way of showing disdain for Lord Naritsugu,When the Shogun insists upon Naritsugu's promotion, Sir Doi seeks out a trusted older samurai, Shinzaemon (Koji Yakusho), who served under the former Shogun. The Shogun's orders will be effective when Naritsugu reaches Edo next.
Sir Doi secretly hires him to assassinate Naritsugu. However, Naritsugu's loyal retainers - led by Hanbei; an old contemporary of Shinzaemon - learn of the plot by spying on Doi's meetings.Naritsugu is so ruthless that he slaughters the entire Mimiya family in cold blood, despite Hanbei pleading with him to leave them alone. But despite the incident, Hanbei is sworn to protect Naritsugu.
Shinzaemon gathers 11 more samurai, whom Sir Doi knows can be trusted including Shinzaemon's nephew, Shinroukuro (Takayuki Yamada). The 12 plan to ambush Naritsugu on his official journey from Edo back to his lands in Akashi.However just before they leave, Hanbei arrives and warns his old colleague that he will suffer grave consequences if he tries to kill Naritsugu.
The assassins decide to ambush Naritsugu at the town of Ochiai, predicting that he will insist on carrying out his Pre-announced visit of Naegi and thus pass through Ochiai. The group, with the legal authority and financial assistance of Sir Doi, buy the help of town Ochiai in order to create a trap.
They also enlist the help of a minor lord Makino who wanted revenge against Naritsugu. When Naritsugu was passing through the town of Kiso the previous year, Makino pressed his entire family in serving the Lord. He ordered his daughter-in-law Chise to carry out a minor task for Naritsugu, but he grabbed Chise by the hair and brutally sexually assaulted her. When Makino's son entered the chambers, he could not confront the Lord, but only comforted is wife, and that's when Naritsugu came up behind him and stabbed him in the back with his sword and decapitated him. The same night Chise killed herself in disgrace.
With troops, he blocks the official highway forcing Naritsugu to head for the town and the trap. During the assassin's own journey to the town, they are attacked by Ronin paid off by Hanbei to kill the plotters.
The group decides to head through the mountains but end up getting lost. In the process they encounter a hunter named Kiga Koyata (Yusuke Iseya) who becomes their guide, and they adopt him as the thirteenth assassin.Makino then commits Seppuku to conceal his involvement in the conspiracy. Sensing a trap, Hanbei advises Naritsugu to avoid Naegi, but Naritsugu refuses as this would publicly embarrass him. Naritsugu acknowledges that visiting Naegi is unsafe, but also exciting.The town is converted into an elaborate maze of booby traps and camouflaged fortifications. Naritsugu and his retinue arrive at Ochiai, with reinforcements arranged by Hanbei and their numbers have been swelled with additional troops.
The 13 assassins were no longer facing 70 men-at-arms as now there were 200. A lengthy battle follows with Naritsugu and his guards trapped inside the village and attacked on all sides by arrows, explosives, knives, and swords - with the exception of Koyata who fights with rocks in slings. Against this carnage, the deranged noble, Naritsugu, is excited by the violence claiming it's the most fun he has ever. He tells Hanbei that when he ascends to the Shogun's counsel, he will bring back the wars of the Sengoku period.
While most of the Akashi forces are defeated, at least 10 of the assassins perish, with several fighting until they collapsed dead from their injuries. Eventually Naritsugu and Hanbei are cornered by Shizaemon and Shinrokuro. Shinzaemon argues that Naritsugu will ruin the realm, but Hanbei insists on loyalty to his master. Shinzaemon duels Hanbei and decapitates him after kicking mud into Hanbei's eyes, while Shinrokuro slays the last two Akashi retainers.
After Shizaemon kills Hanbei, Naritsugu kicks his loyal retainer's head away insulting the older samurai. Contemptuously Naritsugu announces that both the people and samurai have only one purpose and that is to serve their lords. But Shizaemon counters by telling Naritsugu that lords can't live without the support of the people, and that if a lord abuses his powers the people will always rise against them. Naritsugu and Shinzaemon both mortally wound each other. As the lord crawls away in mud crying and experiencing fear and pain for the first time, he thanks Shinzaemon for showing him excitement before Shinzaemon chops his head off.
Shinrokuro wanders through the carnage and Koyata runs up, appearing virtually unharmed despite being impaled through the neck by Naritsugu and slashed in the stomach by Hanbei and appearing to fall dead. Koyata dismisses his previous injuries as trivial. This unusual development and Shinrokuro's amazement and question if Koyata is immortal subtly suggests that Koyata could be a Japanese trickster deity who joined the assassins either to help them, or simply for his own entertainment. Shinrokuro and Koyata leave separately, with Shinrokuro intending to become a bandit or travel to America, while Koyata vows to elope with his lover.
An epilogue states that the Shogun and his government covered up what really occurred announcing that Naritsugu died of illness on the journey back to his lands. In 23 years, the Tokugawa Shogunate would be overthrown with the Meiji Restoration.
