When Commander Monk of the Thames River Police is called to investigate the drowning of an escaped prisoner, he’s forced to contend with customs officer McNab, who clearly bears a bitter grudge against him. But the reason is a mystery in itself. Monk’s memory loss—a secret he guards closely—leaves him vulnerable to repercussions from his missing past, especially his exploits overseas in the tumultuous Gold Rush days of San Francisco. And now McNab, as icy and unfathomable as the steel-gray Thames itself, appears intent on using whatever damning facts he can find to his advantage to ruin Monk’s future as an officer of the law.

As Monk explores the possibility of a conspiracy, McNab’s game of cat and mouse escalates, with veiled threats and cryptic insinuations. Snared in an unforeseen trap, a desperate Monk must turn to his wife, Hester, and friend and attorney Oliver Rathbone for help, as his life literally hangs in the balance.