Casey Bowman (Scott Adkins) is an American orphan who was adopted into a martial arts dojo in Japan. Because of his perseverance and desire to master bushido, he earns the respect of the dojo's sensei, Takeda, and his daughter Namiko (Mika Hijii). However, the dojo's top student Masazuka (Tsuyoshi Ihara) becomes increasingly bitter over Namiko's friendship with Casey until one morning, during a sparring match, he loses his temper and nearly kills Casey by throwing a katana at him. In defense, Casey scars Masazuka below his right eye. As a result of his actions, Masazuka is banished from the dojo by the sensei.
Years later, Masazuka becomes an assassin under contract with an American conglomerate called Temple Industries, which itself runs an underground criminal cult known as "The Ring". He returns to his former dojo and claims in vain the sensei's succession as sōke, but the sensei refuses to oblige. Anticipating an invasion by Masazuka, the sensei sends Casey and Namiko to New York to guard an old chest called the Yoroi Bitsu, which contains the suit and weapons of the ninja who started the dojo.
While in New York, Casey and Namiko seek out a friend of Sensei Takeda, Professor Garrison, to help them hide the Yoroi Bitsu. However, they end of becoming framed for the murder of Garrison and are forced to fight off Masazuka and members of the ring as well as trying to avoid the NYPD. Eventually, the duo are brought in to the station for questioning by the police, but while separated, Namiko is kidnapped by Masazuka.
After escaping from the police station, Casey tracks down a contact number for Masazuka and sets up a meeting, ostensibly to exchange the girl for the Yoroi Bitsu. This sets up a final showdown between Casey and Masazuka.
A sequel Ninja: Shadow of a Tear, also starring Scott Adkins was released in 2013.
Ninja(2009) contains examples of the following tropes:
- The Ace: Casey. It's what Scott Adkins does.
- Action Girl: Namiko, resident bo-staff expert. She's also pretty good with a bow and arrows.
- Arch-Enemy: Casey Bowman has Masazuka, his Rival Turned Evil from his student days, who killed their master and wants to take over their ninja clan.
- Ax-Crazy: Masazuka, full stop. Nearly kills Casey the first chance he gets and slices through his entire dojo to claim the Yaroi Bitsu.
- Badass in Distress: After being kidnapped by Masakuza, Namiko goes from Action Girl to one of these. Justified in that her kidnapper is equally trained and better armed.
- Black-and-White Morality: Played Straight. Casey is the hero and Masazuka is the villain.
- Big Bad: Masazuka, rogue student of the Takeda school. Casey has to go through the Ring to get to Masazuka.
- Chekhov's Gun: The contents of the Yaroi Bitsu are used by Casey in the final fight against Masazuka.
- Conservation of Ninjutsu: Mooks don't stand a chance, but the fight with Masazuka is a good one.
- Cyber Ninja: Masazuka's arsenal is thoroughly modern, including nightvision goggles and submachine guns, but he still prefers his sword. Justified, his services don't come cheap.
- Dragon with an Agenda: An odd heroic example with Casey, who uses his time in New York to search for his missing mother. She died years ago. Played straighter with Masazuka, who calls in a help from The Ring to help capture the Yaroi Bitsu.
- Duel to the Death: It's Casey vs. Masazuka in the final battle with swords and Masakuza loses his head, making it a literal duel to the death.
- Evil, Inc.: Temple Industries, which employs Masazuka to cover up its dirty deeds. The entire Board of Directors is shown to be in on this.
- Gorn: Yep.
- Info Dump: Sensei Takeda explains the history of the Yaroi Bitsu via inner monologue early on in the movie.
- Job Title: It's a movie called Ninja that centers around a heroic ninja.
- Katanas Are Just Better: Masazuka uses some modern weapons, but in his duel to the death with Casey they use katanas.
- Lady of War: Namiko, given the fact that she is an Action Girl who favors a bo staff, is not overly sexualized in the movie, and is the daughter of the revered Takeda.
- Love Interest: Namiko graduates to Casey's wife in the sequel.
- MacGuffin: the Yaroi Bitsu. Justified, as it comes directly down from the founder of the dojo and represents the authenticity of the sensei who possesses it. Plus, its contents are just as deadly as it was hundreds of years ago.
- McNinja: Casey. Averted, as he was raised and trained by a legitimate ninja master in Japan.
- Martial Arts Movie: In typical Scott Adkins fashion...flashy, but no wires.
- Mighty Whitey: Played completely straight. Casey is an American orphaned and raised by a Japanese ninja master. As such, he is trained in the way of the ninja and eventually, after avenging his master, becomes the dojo's sensei.
- Ninja: Averted...just kidding.
- Off with His Head!: Sensei Takeda is beheaded by Masazuka and Masazuka is beheaded by Casey.
- Old Master: Sensei Takeda.
- One-Word Title: It's a film called Ninja.
- Opening Scroll: Gives super brief history lesson on ninjas. Establishes Casey as part of an ancient tradition.
- Plot Armor: Throwing his sword did not work for Masazuka in this movie because Casey was the hero.
- Police are Useless: Masazuka mops the floor with most of a precinct. Casey and Nanako take a few cops out as well.
- Psycho for Hire: Masazuka really enjoys his work.
- A Pupil of Mine Until He Turned to Evil: Masazuka, from Sensei Takeda's perspective. He was expelled from Takeda's dojo for his bad behavior.
- Samurai: Makes an appearance while Sensei Takeda is dreaming of times gone by during his early info dump.
- Stock Ninja Weaponry: The Yaroi Bitsu, again. Masazuka uses more modern variations.
- Sword Fight: Several.
- You Killed My Master: Sensei Takeda's assassination at the hands of Masazuka is what really gets the plot in motion.
- Super Window Jump: Casey makes one of these through the glass in the interrogation room at the police station.
- Would Hit a Girl: Mooks in the movie have no problem hitting Namiko. Justified because she is a ninja herself.
- Zerg Rush: The Ring has plenty of Mooks for Casey and Nanako to run through.