The Continental Op is summoned from San Francisco to Personville by Donald Willsson, who is murdered before the Op meets him. Determined to get to the bottom of Donald's death, the Op learns that Donald's father, Elihu, is losing his control over the city to several competing gangs that he had hired himself many years ago to—ahem—"resolve" a labor dispute.
The Op makes Elihu promise to fork over $10,000 to the Continental Detective Agency and to allow him (the Op) to take whatever steps he thinks is necessary to clean up the city. When the Op figures out the identity of Donald's murderer, Elihu attempts to take back their deal, but the Op won't let that happen and he continues with his mission to clean up the not-so-personable streets of Personville.
At the same time, the Op is also spending a lot of time with Dinah Brand, a possible romantic interest of Donald and a moll for the local gangster Max Whisper Thaler. (Pssst: slang alert! The word "moll" is slang for a gangster's female companion.) Using information from both Dinah and the dishonest Chief of Police, Noonan, the Op spreads information around the city to set off a gang war among the four local factions. Bodies start dropping like flies, as one gangster kills off another and so on and so forth.
After a boozy night of drinking with Dinah, the Op wakes up to find Dinah stabbed to death with an ice pick that he had used the night before. Uh oh. Now the Op is wanted by the police, and he's not even sure if he was the murderer.
The novel culminates in a frantic climax as the Op tracks down the last two living gangsters, Whisper and Reno, who are facing each other at gunpoint. Reno kills Whisper but is badly wounded in the process. Reno confesses to stabbing Brand, but that when she fell, she collided into the half-unconscious Op, landing in such a way that made it look as if the Op had killed her. By the end of the novel, all the gangsters have killed each other off, and the Op has completed his mission to rid Personville of its crooked gangs.