San Francisco, 1878. Ah Sahm, a newly arrived Chinese immigrant with serious fighting skills, is introduced to Chinatown's most ruthless tong, the Hop Wei, by Chao, a fixer. After impressing Young Jun, son of tong leader Father Jun, Ah Sahm is branded and taken to a brothel, where he befriends Ah Toy, a courtesan with connections.

Later, in search of a woman who left China two years earlier, Ah Sahm crosses paths with Mai Ling and Li Yong, followers of the rival tong leader Long Zii, who is trying to avoid an opium war with the Hop We - a war that Walter Buckley, deputy to San Francisco Mayor Samuel Blake, actively promotes. Meanwhile, after two Chinese laborers are killed by white thugs, police sergeant "Big Bill" O'Hara is tasked with creating a Chinatown squad. Enlisting southern-born cop Richard Henry Lee, Bill soon finds that the hostility between the Chinese and white dock workers, whose unofficial leader is Irish tough Dylan Leary, is unlikely to end anytime soon.

Leary pressures gentleman industrialist Byron Mercer, who is Penny's father, to hire his men for a cable-car track job - despite the fact that Mayor Blake hasn't yet awarded Mercer the contract. Accused of assault and perhaps worse, Ah Sahm gets a cold shoulder from the Hop Wei, with his fate in the hands of an unexpected ally. Buckley urges Mai Ling to eschew restraint and start a war with the Hop Wei.