A series of murders is being carried out by someone claiming to be Jack the Ripper. In response, a vigilante group of the town's poor confronts Scotland Yard while William and the real Jack scramble to uncover the true culprit's aims and foil their plans.

A culprit is arrested in the Jack the Ripper case which was solved by the Moriartys. Suspecting a frame-up by Arterton, Lestrade asks Sherlock for help in finding evidence that would prove it. Meanwhile, Paterson works to sneak Bonde inside the station.