1903. A darkness has descended on London. A series of grisly murders are uncovered, trophies taken, bodies arranged and soon there are whispers of Jack the Ripper's return.

A new client arrives at Baker Street seeking Sherlock Holmes's help: Dr Jekyll claims his friend has been wrongfully accused of the hideous crimes, a friend called Mr Edward Hyde, whose very existence relies on a potion administered by the doctor himself.

But the case becomes more complicated, more unsettling than simply proving Mr Hyde's innocence - for Holmes and Watson unearth beastly transformations, a killer who moves unseen, a secret organisation and then find a traitor in their midst....