When she is hired to investigate devil worshippers performing human sacrifices in San Francisco, private detective Sharon McCone discovers what her clients are really up to and goes undercover to find the truth.

1890s San Francisco. Former Pinkerton operative Sabina Carpenter and her detective partner, ex-Secret Service agent John Quincannon, tackle two seemingly unrelated cases that are complicated by two murders and the interference of Sherlock Holmes.

For the firm of Carpenter and Quincannon, Professional Detective Services, stopping extortionists is not only grand, but excitingly lucrative. When a pleasant afternoon's bicycling through Golden Gate Park with a friend ends with the revelation of threatening letters, followed by a gunshot in a mansion garden, Sabina Carpenter knows this is a case that demands her immediate and undivided attention.The questions her partner John Quincannon has to unravel...