American martial artist Casey Bowman has settled down at the Kōga ninja dojo and married Namiko Takeda, who is pregnant with their first child. One day, while shopping for a pendant in town, he encounters and fends off against two knife-wielding muggers. Later that night, Casey goes grocery shopping, but when he forgets that the muggers took his wallet, he rushes home only to find Namiko slain, with markings of a barbed wire weapon around her neck. On the day of the funeral, the dojo is visited by a former student named Nakabara, who offers Casey to train at his dojo in Thailand to ease his pain, but Casey declines the offer. Remembering the fighting style of the muggers he encountered, Casey heads to the Azuma dojo to know their whereabouts before ambushing and killing them in a dark alley.

Days later, Casey takes Nakabara's advice and travels to Bangkok. While sparring with Lucas, one of Nakabara's students, Casey suddenly loses his temper and assaults him with a bokken. Nakabara reminds him to control his emotions by having him undergo the fire walk practice, but Casey stops halfway through the walk due to memories of Namiko lingering in his mind. He goes on a drinking binge at a nearby bar and gets into a fight with several drunk guests. The next morning, Lucas is killed by the same barbed wire weapon used on Namiko. Nakabara reveals to Casey that his father and Sensei Takeda, along with a man from Nagoya named Isamu, were the top students of the Kōga dojo. When their sensei died, Isamu challenged Takeda for control of the dojo; Takeda killed Isamu in the fight and continued as sōke. Isamu's younger brother Goro witnessed the fight and swore to avenge his death, even if it took three generations. Years later, Goro became head of one of the largest drug cartels in Myanmar. Nakabara urges Casey to return to the U.S., as being Takeda's son-in-law has made him a target of Goro. Casey instead asks Nakabara to help him find Goro. Nakabara gives him an old map of Burma from his father's days in World War II, with markings that indicate locations of ninja weapons.

Casey heads to Myanmar, where he befriends a cab driver named Mike. Later that night, he enters a bar and fights a group of drug dealers. He returns to his hotel room to rest, only to find himself arrested by the SPDC, who accuse him of being an American spy and torture him. He escapes from his cell and extracts information on Goro's whereabouts from SPDC General Sung before Mike drives him to the jungle. There, Casey finds the cemetery of Japanese soldiers and arms himself with a boxful of ninja weapons from under a tombstone marked with the Nakabara clan symbol. He sneaks into Goro's hideout, setting the complex on fire before facing Goro's right-hand man Myat. The fight ends with Casey stabbing Myat in the heart and breaking his neck. He then squares off against Goro before slashing him in the midsection. In the middle of the fight, Goro wraps his barbed wire manriki around Casey's neck, but Casey uses his strength to free himself and throw Goro to the ground before decapitating him.

Casey returns to Nakabara's dojo, only to discover that Nakabara was the one who murdered Namiko and Lucas. Nakabara is revealed to be a drug lord himself, and he used Casey to wipe out Goro's cartel to monopolize the Southeast Asian drug trade. He then gives Casey the choice to either join him or die. Both men engage in an intense fight until Casey kicks Nakabara through a thin wall, revealing a room full of ancient artifacts. They continue the fight in the room with weapons, with Casey slashing Nakabara in the midsection and Nakabara impaling Casey's left shoulder with his katana. Nakabara lunges toward Casey, but Casey grabs a manriki and wraps it around Nakabara's neck for the kill. Later, Casey reveals Nakabara's true motives to Toji, one of the dojo's students. Knowing that such actions would ruin the dojo's reputation, Toji states the incident never happened and bids Casey farewell. Casey returns to Japan and drops Namiko's pendant in a pond, bringing closure to his loss.