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 a anonymous blackmailer who's out to ruin Lacey any way he can.