In the dark and slimy streets of Wapping a prostitute is beaten half to death, a not uncommon fate in late 18th century London. So begins this gripping tale set in 1798 in the Port of London: a cruel villain holds sway over the underworld. His name is Boylin. His face is scarred by lime and his back by the two hundred lashes he received following a naval court martial. He holds Captain Tom Pascoe responsible for his suffering. They meet again when Pascoe becomes River Surveyor for the newly formed marine police. They've had orders to investigate a sudden fall in government revenue that is affecting the nation's ability to fight the war against Napoleon and stem the rising tide of Irish rebellion. Pascoe knows that Boylin is behind it, but he can't prove anything, yet.