Inspired by an idea from martial-arts legend Bruce Lee, "Warrior" is a gritty, action-packed crime drama set during the brutal Tong Wars of San Francisco's Chinatown in the second half of the 19th century. The series follows Ah Sahm (Andrew Koji), a martial arts prodigy who emigrates from China to San Francisco under mysterious circumstances. After proving his worth as a fighter, Ah Sahm becomes a hatchet man for the Hope Wei, one of Chinatown's most powerful tongs (Chinese organized crime family).

Befriended by Wang Chao (Hoon Lee), a fixer and profiteer in Chinatown, Ah Sahm is taken under the wing of Young Jun (Jason Tobin), the hard-partying son of the Hop Wei's leader, Father Jun (Perry Yung), who is facing an imminent opium war with a rival tong. As he is indoctrinated in the rules of Chinatown gangs, Ah Sahm wins the confidence of a canny brothel madam, Ah Toy (Olivia Cheng), revealing to her the true intentions of his journey to America.

Over the course of the season, Ah Sahm crosses boundaries inside and outside Chinatown, bringing him into contact with tong rivals Mai Ling (Dianne Doan), a beautiful and ruthless Chinese woman who has achieved a position of power in one of the tongs, and Li Yong (Joe Taslim), a tong lieutenant and Gung Fu master. As violence erupts, Sergeant "Big Bill" O'Hara (Kieran Bew) and Richard Henry Lee (Tom Weston-Jones), a transplanted southerner and rookie cop, are tasked with forming a Chinatown squad.

Meanwhile, working-class leader and tough Irishman Dylan Leary (Dean Jagger) is fighting for all Irish laborers by trying to inflict as much damage as possible on the Chinese immigrants he says are stealing their jobs. Amidst the strife, power-hungry San Francisco Mayor Samuel Blake (Christian McKay) and Deputy Mayor Walter Buckley (Langley Kirkwood), who has his own political aspirations, engage in widespread corruption, while the mayor's young wife, Penny (Joanna Vanderham), finds herself trapped in a loveless marriage.