Jacob’s Island is a broken-down waterside slum, a place of perpetual gloom where Monk is just as likely to lose a man to the mud and tides as to the criminals who hide in the shadows. Kidnappers have chosen this warren of tunnels and alleys as the site of a ransom exchange for Kate Exeter, the wife of wealthy real estate developer Harry Exeter. Harry asks the Thames River Police to secretly accompany him to the spot and ensure that no harm comes to him or his captive wife. But on arrival, Monk and five of his best men are attacked from all sides, and Monk is left wondering who could have given away their plans.
As Monk follows leads from Kate’s worried cousin and a crafty clerk at the bank where Exeter gathered the ransom money, it seems undeniable that one of his own men has betrayed him. Delving into their pasts, he realizes how little he knows about the people he works with every day, including his right-hand man John Hooper, the one colleague Monk has always been certain he can count on—but who is also hiding a dreadful secret. As the case runs into a series of deadly obstructions, Monk must choose between his own safety and the chance to solve the mystery—and to figure out where his men’s loyalty really lies.