London, 1865, and young Theresa Hatch (Tess, to her friends) receives a nast surprise late at night. When Horatio finds a young girl on his doorstep, passed out, dying - apparently poisoned - he's appalled. Investigations lead to Tess's old workhouse, but a surprise visit to that sorry establishment yields more questions than answers. Only one thing is clear: something very, very bad is happening to the children in the East End.

There's a mystery to be solved, sending Lyle, Thomas, Tate and - naturally - Tess out into the wilds of east London and a certain former thief's old stamping grounds. What they find is terrifying: Tess's old crowd of artful dodgers and ace pickpockets are now wandering the streets like zombies, drooling in the workhouses or plain mad in the asylum. And it isn't just affecting Tess' old crowd; children all over the area are turning up with their memories in tatters and their minds all but gone. The only clue is a name, half-whispered in fear: Old Greybags.