On the bustling docks along the River Thames, Great Britain's merchant ships unload the treasures of the world. And here, in dank and sinister alleys, sex merchants ply their lucrative trade. The dreaded kingpin of this dark realm is Jericho Phillips, who seems far beyond the reach of the law. But when 13-year-old Fig is found with his throat cut, Commander William Monk of the River Police swears that Phillips will hang for this abomination.

Monk's wife, Hester, draws a highly unusual guerrilla force to her husband's cause - a canny ratcatcher, a retired brothel keeper, a fearless street urchin, and a rebellious society lady. To one as criminally minded as Phillips, these folks are mere mosquitoes, to be sure. But, as he will soon discover, some mosquitoes can have deadly stings.