On an evening in April 1906, Patrolman Jerry Boyne of the San Francisco Police Department is walking his beat on Nob Hill when suddenly a horse-drawn cab hurtles past him and down the steep Mason Street hill. Fearing a certain smashup, he rushes to follow the cab. When a scream and a cry for the police diverts him to the Claiborne mansion, he discovers that four-year-old Jamie Claiborne has been kidnapped and his nurse-maid murdered. Thinking that this case will be what he needs to advance up the police ranks, Jerry immediately begins investigating. However, he is soon frozen out of the investigation when his captain, San Francisco's political boss, and Hard Knox Gahagan, the youngest police commissioner in the city's history, arrive to take over. Jamie's father, James G. Claiborne, is convinced that his older daughter, Leonora, is behind the plot and declares that he will not be shaken down by her and her accomplices. At the urging of Norah, the Claiborne's house-maid, with whom Jerry is in love, and believing that the runaway cab is a key to the case, he decides to conduct his own investigation and soon ends up on the wrong side of the law. Now he has both the kidnappers and the police after him. Just as he is about the break the case wide open -- and expose some of San Francisco's most powerful men -- the earthquake strikes and the city itself is literally shaken down.