It's been a year since a body found floating in the murky waters of the Thames thrust penniless Joe, Edie, Ottilie and Jack into the criminal underbelly of Victorian London.

The Mudlarks, now raised from poverty, are pursuing their own ventures. But their new-found security is shattered when Jack crosses paths with a fearless street urchin called Flea.

Flea knows the whereabouts of a deadly treasure hidden deep beneath London, and he needs their help to recover it. Yet they are not the only ones on the trail of the treasure, and the dark sewers beneath Hampstead hold forces more sinister than anything the Mudlarks have faced before.

As they face criminals, cutthroats and things far worse, familiar faces will force Jack to confront memories he had struggled to forget, and the Mudlarks will fight to survive on their most monstrous adventure yet.