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 Wei, 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.

Intercepting a shipment of opium at the docks, Young Jun, with Ah Sahm and his Hop Wei lieutenant Bolo in tow, decides to send a message to Long Zii. Big Bill and Lee investigate a grisly murder scene in an alley next to an Irish bar, The Banshee. Penny Blake, the young wife of San Francisco's mayor, finds herself in a bind while visiting the wharf with her Chinese manservant, Jacob. Ah Sahm pays a steep price for playing the hero. The Long Zii clean up a mess, and brace for more bloodshed.

Big Bill discovers a possible solution to his gambling excesses; Penny reveals the circumstances that prompted her to marry Mayor Blake; Mai Ling offers Ah Sahm a way out of a protracted and bloody tong war.

Ah Sahm's scheme to protect Penny's factory while serving his own covert Hop Wei agenda seems to be working well, but Leary has other plans for the factory that will change things drastically.