The setting is Feudal Japan, 1844. Lord Naritsugu is the younger brother of the shogun, which affords him the unchallenged right to murder, rape, torture, and mutilate on a whim. After a high-ranking official protests Naritsugu's latest outrage by publicly committing Seppuku, a conspiracy is formed within the shogun's senior council to eliminate Naritsugu before his madness plunges the entire nation into anarchy. A veteran samurai named Shinzaemon is recruited to accomplish the suicidal task, assembling a ragtag group of twelve warriors (including one enigmatic vagrant) for one last chance at an honorable death in an age of peace. They quickly transform a sleepy little village into a booby-trap laden killing gound, preparing to ambush their target before he reaches the safety of his estate. But even with surprise and skill on their side, can the small group of assassins possibly hope to prevail against Naritsugu's private army of more than 200 soldiers? In a word? Yes.
- Asshole Victim: Naritsugu. Definite case of "He had it coming."
- Badass: All of the assassins, but especially Hirayama (who kills a bunch of Naritsugu's guards with an entire alleyway full of swords) and the vagrant Kiga (who takes down plenty more guards with nothing but a sling and a few impromptu clubs).
- Badass Boast: Sahara calls out Natisugu and his guards by unrolling a scroll with the words "Total Massacre" written on it, by Naritsugu's former sex toy no less. They succeed in fulfilling this boast.
- Big Badass Battle Sequence: One that continues unbroken for over 40 minutes.
- Biggus Dickus: Apparently, Kiga has a go with so much women he leaves them all spent (looking as if they're dying). And that even isn't enough for him and takes the village elder straight in the ass.
- Blade on a Stick: Sahara's yari.
- The Caligula: Naritsugu.
- Combat Pragmatist: In training, Hirayama (who studied under Shinzaemon) instructs the others to win a fight by any means necessary, and to inflict as much damage as possible in case they die. Shinzaemon himself wins his final duel with Hanbei by kicking mud in Hanbei's face and attacking while he's distracted.
- Complete Monster: Naritsugu embodies this trope, and is considered such In-Universe, a fact which drives the main plot. Among other things, he uses an entire samurai family as target practice (including their young daughter) and slices off all of a peasant woman's limbs to use her a sex toy. To quote this review of the movie: "Magnitude 8.3 on the Dickter Scale."
- Conservation of Ninjutsu: Twelve of the thirteen are either more experienced than most of the army, or were trained by someone who is (Kiga, the thirteenth, is just a brawler, and probably isn't even human). Hanbei provides far more of a one-on-one challenge than any of his troops. Kiga lampshades this during the final battle:
"I thought samurai would be fun! But you bore me. You're useless. Even more useless in great numbers."
- Deliberate Injury Gambit: Shinzaemon's final duel with Naritsugu, resulting in a Mutual Kill. Played with in that, given Shinzaemon's stated motivations going into the battle and his obviously superior skill level, it's clear that receiving a fatal injury was an end unto itself for him, rather than a means to an end.
- Driven to Suicide: Multiple instances of Seppuku, including one which kicks off the whole plot.
- Due to the Dead: Naritsugu earns more bastard points by kicking Hanbei's head like a soccer ball after Hanbei had given his life to protect Naritsugu. When Shinzaemon calls him on this, Naritsugu invites him to do the same to his own head.
- Dwindling Party
- Face Death with Dignity: The samurai use this as a code of honor. Guess who didn't face his death with dignity?
- Foreshadowing: Early in the film, Hirayama instructs the lesser samurai: "No sword? Use a stick. No stick? Use a rock. No rock? Fight with your fists and feet." He and his student end up using those exact tactics in the final battle.
- For Great Justice
- Friendly Enemy: Shinzaemon and Hanbei were students together, and greatly respect each others' abilities.
- Go Out with a Smile: Kuranaga.
- Gut Punch: The first of the deaths out of the 13 is the signal that things are about to get very, very ugly.
- Improbable Aiming Skills: Kiga. There may be a very good reason for this.
- Incendiary Exponent: Stampeding bulls. On fire.
- Infant Immortality: Played with in Naritsugu's "target practice" scene: his last remaining target is a little girl, but he keeps missing her. Eventually averted.
- Kick the Dog: Happens just about every time Naritsugu is on screen, including kicking the severed head of his most loyal subordinate.
- Kill'Em All: Only two of the thirteen survive the battle at the end of the film.
- Master Swordsman: Hirayama, Shinzaemon, Kuranaga and Hanbei.
- Mass "Oh Crap"
- Maybe Magic, Maybe Mundane: Kiga, the thirteenth assassin, is an ex-bandit who gets kicked out of his crew for touching his boss's wife. Or so he claims. The point of view of his flashback to the event, his Unexplained Recovery moment, and the first supposition of the first twelve assassins indicate he might be a Tanuki or a Kappa. In an interview, the director also presented the possibility his appearance after the battle might be his ghost, but said that it's up to each viewer to decide what happened to him.
- My Master, Right or Wrong: Hanbei knows his lord is evil but is willing to fight to the death to protect him.
- Nightmare Fuel: Shinzaemon's reaction when he meets the "poor woman". But this also gives him the resolve to fulfill his mission.
- Off with His Head
- Rated "M" for Manly
- Ronin: Hirayama and Sahara.
- See You in Hell
- Suicide Mission: None of the samurai expect to walk away from their assassination attempt on Naritsugu. Many of them are even counting on it, seeing this as their last opportunity to find a warrior's death on the battlefield in this era of peace.
- Tanuki: Probably Kiga.
- Throwing Your Sword Always Works: Naritsugu takes out the thirteenth assassin this way. Well, it puts the guy down for the rest of the battle, at least...
- Unexplained Recovery:
- For all the explosions, elaborate traps and badassery, there's nothing to suggest any fantasy elements until Kiga, the 13th Assassin survives despite having a sword shot at his neck...and shows up after the battle, completely unharmed. The viewer is left wondering just who or what he is.
- What Is This Feeling?: Pain, for Naritsugu.
- Worthy Opponent: Hanbei and Shinzaemon view each other as such.
- You Shall Not Pass
- Your Approval Fills Me with Shame: During the assassination attempt, Naritsugu comments to Hanbei that he will use his position to start more wars because the fighting is so awesome. He also thanks Shinzaemon for giving him such an exciting day.