Hanover Square is one of the wealthier neighborhoods of London, certainly not an area where one would expect a riot. However, Captain Lacey spies a mob howling for blood before a group of cavalrymen disperses them - and the leader of the rabble is felled by a bullet in the back.

The wounded man, Thornton, believes his daughter was kidnapped right off the street and is being held captive against her will in 22 Hanover Square, occupied by one Josiah Horne. Looking into the matter, Lacey involves himself not only in the mysterious disappearance of a beautiful young woman, but also the sinister endeavors of an enigmatic crime lord, and the murder of a country squire's maid...