Captain Lacey is asked by Peter Thompson of the Thames River Police to help him investigate a cold case–the murder of a woman found near the docks Thompson patrols. The investigation was sidelined, considered unsolvable, but Thompson has long wished to find her killer.

Captain Lacey joins him in the hunt, entering a part of society that is closed to outsiders. Meanwhile, he must deal with his daughter's debut and more developments in his new domestic life, including an anonymous blackmailer who's out to ruin Lacey any way he can.