On a foggy night in 1850, Mary Rutledge (Hopkins) and retired Colonel Marcus Aurelius Cobb (Frank Craven) arrive in San Francisco Bay aboard the clipper Flying Cloud. She had come to wed a wealthy owner of a gold mine, but it is revealed that he had lost his mine when the roulette wheel landed on red 13 times at the Bella Donna. The men at the wharf reluctantly inform her that her fiancé is dead, murdered most likely by Louis Chamalis (Robinson), the powerful owner of the Bella Donna restaurant and gambling house. Mary is upset, but quickly pulls herself together and asks the way to the Bella Donna.
Mary meets Chamalis and agrees to be his companion, not only for business reasons (as an attraction, she helps draw in customers), but for personal pleasure as well. Chamalis gives her the name ‘Swan’, and she becomes his female escort. She accompanies him on promenades in town, and he showers her with extravagant gifts. Their relationship sours quickly because Swan is angered by Chamalis’s destructive power-mongering. She does not, however, mind running a crooked roulette wheel and cheating the miners out of their gold.
Colonel Cobb purchases a printing press, with the intention of starting a respectable newspaper for the people of San Francisco. His first issue includes an article criticizing an unpunished murder by Chamalis and his entourage. When Chamalis finds out, he threatens to destroy Cobb’s printing press and burn down the building, but is halted by Swan. Chamalis demands that Cobb never print anything attacking him. The colonel unwillingly complies.
Swan becomes disillusioned with her life in San Francisco. Her distant behavior irks Chamalis. One morning, she sets out on horseback. When it begins to rain heavily, she seeks refuge in a seemingly abandoned cabin, where she meets poet and gold miner Jim Carmichael (McCrea). Swan is taken with him, but lies about her current situation after hearing his criticisms of the city. He gives her his book of poems as a memento.
Carmichael decides to return to New York. Because of fog the ship will not leave for a few days. He meets Chamalis’ helper, Old Atrocity (Walter Brennan), who, seeing his bags of gold is happy to show him to the Bella Donna. Carmichael is surprised to find Mary working there. He is served drugged liquor and plays roulette at her table. He loses his composure, insults ‘Swan’ and eventually loses his money.
Carmichael wakes the following morning in the Bella Donna’s kitchen. His eloquent speech impresses Chamalis, who hires him on the spot as a waiter. Carmichael’s presence perturbs Mary, who offers him money to depart. Carmichael refuses, wishing to earn the fare on his own.
Cobb puts up a poster telling about a murder Chamalis ordered and how the Bella Donna cheats customers. Seeing it, Chamlis’ henchman “Knuckles” Jacoby (Brian Donlevy) shoots both the man who put it up and the publisher when he tries to defend him. Dying, Cobb orders his assistant to print the truth. A vigilante group is formed and hangs Knuckles.
Devastated by Cobb’s death, Mary acknowledges her love for Carmichael, and works the roulette table so that he wins back the gold he previously lost. Chamalis finds out and sets out to kill Carmichael, who has snuck into Mary’s bedroom. The lovers decide to leave together. They find a rowboat and attempt to board the ship in the harbor. They have trouble seeing in the fog, but can hear Chamalis pursuing them. He shoots and injures Carmichael, and corners them beneath a pier. Mary begs him, as proof of his love for her, not to kill Carmichael. Chamalis agrees, but tells her he does not want her anymore. The sheriff arrives with a mob, and Chamalis allows himself to be taken away. Mary returns to Carmichael’s side aboard the ship as it prepares to set sail.