On May 26, 1889, four thousand mourners proceeded down Chicago's Michigan Avenue, followed by a crowd forty thousand strong, in a howl of protest at what commentators called one of the ghastliest and most curious crimes in civilized history. The dead man, Dr. P. H. Cronin, was a respected Irish physician, but his brutal murder uncovered a web of intrigue, secrecy, and corruption that stretched across the United States and far beyond. O'Brien tells the story of Cronin's murder from the police investigation to the trial-- and the story of a booming immigrant population clamoring for power at a time of unprecedented change.