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 remembers 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 find out 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 patrons. 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 (Namiko’s father), along with a man from Nagoya named Isamu, were the three 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 United States, as being Takeda’s son-in-law has makes him one of Goro’s targets. Instead, Casey asks him to help him find Goro. Nakabara gives him an old map of Burma from his father’s days in World War II, with markings indicating locations of ninja weapons.