In Japan, American-born Aiko Saito convinces her Japanese husband, Akira, to accept a business opportunity in the United States. Before leaving, Akira Saito visits Koga, the priest who adopted him and his orphan brother, Shoji. In the past, Akira Saito found Shoji stealing from the temple, and accidentally killed him. Koga advises Akira to work through the guilt or it will consume him. Akira refuses to take a sword that Koga gives him as a keepsake, but accepts a ninja warrior helmet instead. Koga insists that he cannot escape his destiny, and Akira swears that he will keep his ninja identity a secret until death. Akira and Aiko relocate to Los Angeles, California, with their sons, Takeshi and Tomoya. They purchase a hotel and restaurant property from Sam Green in a shabby part of town. As Sam shows them the premises, he does not realize that a ring of jewel thieves is using a shuttered tobacco shop on the property to hide their loot. Later, one of the thieves pretends to place an expensive Van Atta diamond necklace under the floorboards, but pockets the jewelry instead. As Sam leaves to start his retirement in Atlantic City, New Jersey, the thieves force him to detour to a deserted part of town because they believe he stole the Van Atta necklace. When Sam claims to know nothing about it, they beat him unconscious and set him and his car on fire. Gang leader Mr. Newman confronts associates about the missing necklace, but none know its whereabouts. As Takeshi Saito fights a gang of boys to prevent them from stealing the brothers’ bicycles at the corner market, Limehouse Willie, one of Mr. Newman’s hoods, kidnaps Tomoya, the younger boy. As Takeshi returns home with a bloody face, the hoods telephone and demand that Akira Saito return the Van Atta necklace to Pier 24 at midnight or Tomoya will be killed. Akira finds some clues when he searches the former tobacco shop. At the pier, Newman’s hoods handcuff Akira, who reveals that the man who took the necklace wore a gray suit and was left-handed. Limehouse Willie does not believe him. He threatens to burn Tomoya with an acetylene torch, but father and son employ their martial arts skills to escape. Mr. Newman orders his men to execute Akira. At the police station, Akira identifies Limehouse Willie from a photograph and agrees to help police catch the gangster if they protect his family, but a corrupt officer named Sergeant Daley passes the news on to Newman. As Tomoya and Aiko chase a soccer ball in the street, Newman’s thugs hit them with their car. After mother and son leave in an ambulance, Akira recognizes Newman’s men, who have returned to the crime scene, and chases after them. He climbs onto the back of the truck and causes the driver to crash, killing Newman’s men. At the hospital, Aiko is badly bruised, but will survive. When Akira returns to the waterfront to confront Limehouse Willie, he knocks out two-dozen men, cuts the power, and demands that the gang stop intimidating his family. When the lights come back on, Akira is gone. Limehouse Willie vows revenge. He disguises himself as a doctor and kills Aiko in her hospital bed, but is chased away before he can administer a fatal injection to Tomoya. When Lieutenant Anderson warns Akira not to take matters into his own hands, the enraged widower dares police to stop him. Meanwhile, Limehouse Willie puts the clues together, kills Sgt. Daley as he attempts to sell the Van Atta necklace, and takes possession of Daley’s briefcase full of cash. Akira hides his sons in a warehouse near Pier 24, where he practices his ninja skills and forges a special sword. Then, after stealthily entering Mr. Newman’s compound, he overpowers the security staff with his sword, arrows, razor-sharp metal projectiles, and martial arts skills, despite being shot in the thigh. Finding Newman, Akira impales him through the throat. Limehouse Willie grabs the money, escapes in a pickup truck, and tries to run Akira over, but the ninja fighter hides underneath the chassis. Limehouse Willie drives to Akira’s warehouse hideaway to kill the boys, but as he grabs an axe, Akira emerges and breaks the weapon during hand-to-hand combat. After a chase through a neighboring warehouse, Akira drives a metal stake through Limehouse Willie, nailing him to a log resting on a band saw. As Akira turns on the machine, Limehouse Willie begs to be killed as the log moves toward the spinning blade. Later, Akira and his sons visit Aiko’s grave. Lt. Anderson joins them with the news that Mr. Newman and his associates were wiped out in a gangland killing, and the Federal Bureau of Investigation will conduct an inquiry into Sgt. Daley’s involvement. Witnesses report that a ninja was responsible for the massacre, but Akira responds that ninjas do not exist. The lieutenant suggests that if Akira crosses paths with the ninja, let him know that police do not want him causing any more trouble in the area. Anderson tosses a metal projectile at Akira, who smiles and bows his head.