In Feudal Japan the era of the samurai is waning. The sadistic young Lord Naritsugu rapes and kills at will. No one can touch him because he is the Shogun’s younger brother and thus above the law. A senior government official realizes the situation will become more dangerous after Naritsugu ascends to a higher political position. After a wronged party publicly commits seppuku, a trusted older samurai, Shinzaemon, is secretly hired to assassinate him.
He gathers eleven more samurai, including his nephew Shinroukuro, and the dozen plan to ambush Naritsugu on his long journey home from Edo. Before they leave, they are visited by Naritsugu’s samurai Hanbei, an old sparring partner of Shinzaemon’s: each tries to dissuade the other from his task in vain, and Hanbei leaves promising to fight later.
Several of the assassins are sent to buy the help of a town to block Naritsugu’s passage in order to force his route to their advantage. Shinzaemon and the others ride urgently towards the town they have chosen for the battle. On the way, they are met with a few of Naritsugu’s men who they kill easily. They decide to go travel stealthily through the mountains, where they meet a hunter named Kiga Koyata who at least claims to be of samurai lineage, and they adopt him as the thirteenth assassin. He helps them find the village they have chosen for the conflict, and they are reunited with their comrades. They take the village over and convert it into a labyrinthine mousetrap with many camouflaged fortifications.
But when Naritsugu arrives, the assassins discover they are outnumbered even more heavily than originally expected: there are 200 guards, not seventy. Nonetheless they decide to continue their mission. The lengthy battle follows, with the guards and Naritsugu trapped inside the village and attacked by the assassins with arrows, explosives, knives, and swords — with the exception of the Koyata, who fights with rocks in slings. Naritsugu, who has led a life of luxury, is enthralled by the violence. Inevitably, the assassins fall one by one while they inflict devastating casualties on the guards. Until, at last, there is a one-to-one sword fight between Shinzaemon and Hanbei, with Naritsugu watching. Shinzaemon triumphs, and then fights with an excited Naritsugu, who dies in fear and pain in the mud. Shinzaemon is mortally wounded by his fights, leaving only his nephew and the hunter standing.