A Japanese student named Watari Katsunoshin travels to the United States during the tumultuous 1800s, an era known as the Wild West in America and the Meiji period in Japan. After arriving for his studies, he finds himself penniless and is forced to work as a gold hunter to survive. During this time, he meets a beautiful Native American woman named Sakura, and the two have a son together named Isamu. Watari and Sakura agree to travel to Japan together, but Sakura suddenly passes away. Watari then falls victim to bandits, loses the young Isamu, and is left brutally injured.

Isamu ends up stranded alone on a gold panning site where the workers take pity on him and raise him as their own. Years later, a deadly storm kills all the workers, leaving Isamu as the sole survivor. Alone and with hope lost, he is discovered by a feared outlaw named Wingate and his son. Wingate takes Isamu in and teaches him how to use weaponry, believing the boy has potential as an assassin. Despite feeling deeply indebted to Wingate for saving his life, Isamu cannot bring himself to kill anyone due to his inherently good nature. This moral conflict becomes a central struggle as Isamu journeys across the American frontier, facing outlaws at every corner, corrupt governors who oppress their own people, deep poverty in the cities, and the continued existence of slavery.

A significant figure in the story is Big Stone, a menacing African American outlaw who initially serves as an antagonist to Isamu. Big Stone's motives are noble; Wingate killed his mother, and he uses his skills to free enslaved African Americans. This earns him Isamu's respect, and he becomes a worthy opponent who leaves a lasting impact on the boy. The narrative also features appearances by historical figures such as Wyatt Earp, Billy the Kid, and Doc Holliday. Throughout his coming-of-age journey, Isamu struggles with conflicting loyalties between his adoptive father and the innocent people Wingate has harmed, while holding onto the dream of finding his long-lost father. The story continues until a final reunion between father and son.