In turn-of-the-century San Francisco, Jack London--who is just starting his career as a writer--supplements his meager income by moonlighting for the police. One night, Jack is hired to join Officer Healey on a stakeout. One of the men they are watching is safecracker Lou Kandel, a recent escapee from San Quentin. Jack and Healey get into an altercation with Kandel and another thug but they both escape. When Healey is murdered the next day, SFPD brass asks Jack, who is able to identify Kandel's crony from mug shots, to stay on the case because they believe that a master criminal is planning to steal the Rajah's Ruby from the visiting "Belle of Broadway," the sexy Belle Conquest.