Responding to an urgent summons, Dr. Watson arrives at 221B to find Sherlock Holmes swathed in filth and in a state of deep shock. To ensure Holmes' safety, he must retrace the detective's last movements and discover the source of his debility. The investigation leads to a fire-gutted warehouse where nothing remains except an open cesspit and the enigma of an unburned circle.

But answers are not forthcoming and he must rely on a convalescing Holmes, who unravels the mystery by relating his encounter in the riverside warehouse with a time traveler. With a lack of physical evidence and only Holmes' account of what transpired, Watson must determine if the related events are real or a fabrication of Holmes' fevered mind.