A superintendent in the Thames River Police, William Monk is on a patrol boat near Waterloo Bridge when he and his men notice a young couple standing at the bridge railing, apparently engaged in an intense discussion. The woman waves her arms and places her hands on the man's shoulders. A caress or a push? He grasps hold of her. To save her or to kill her? Seconds later, the pair plunges to death in the icy waters. Monk can't help but wonder, has he witnessed an accident, a suicide, or a murder? It seems impossible to determine the truth but, haunted by the woman's somber beauty, he is impelled to try.

Mary Havilland is her name and until recently she and Toby Argyll, the fair-haired man who shared her fate, had planned to marry. Her father, an engineer employed by the Argyll Company, has also recently died - a suicide according to the police and Mary's sister. But friends tell Monk that Mary suspected her father had been murdered because of his stubborn insistence that the Argyll Company's current project - participation in the construction of a splendid new sewer system for the metropolis - is so badly flawed that the entire city may be in dire peril from flood and fire.

Already struggling to win the respect of his men, Monk is faced with two urgent mysteries. With his intrepid wife, Hester, he is soon treading a slippery path - from luxurious drawing rooms where powerful men hatch their unscrupulous plots to a world beneath the city where poor folk fight starvation. In nightmarish tunnels, Monk and Hester find true friends, among them Scuff, a young mudlark; Sutton, the ratcatcher, and Snoot, Sutton's clever terrier. For once, even Monk's old enemy, Superintendent Runcorn, is on his side. But as rainfall strains the fragile manmade underground, a murderer remains free - and poised to strike again.