The River Rat Murders presents the first part of the diary of Caroline Case, a Wabash Valley Madame, during Prohibition. After the murder of her friend Alec in 1921, Caroline becomes an amateur but extremely resourceful detective. She and her mysterious new friend, Hannibal Jones, her "girls" and an odd assortment of local characters solve five murders. They get no help from the authorities, who are on the take from gangsters who control the booze trade in the Valley. Caroline, Hannibal and their friends call themselves "The River Rat Detective Agency." As they solve the murders, they become involved in the war between Al Capone and the North Side gang over control of the booze trade. There are shootouts, kidnappings and daring rescues.