Dr. Watson, a military surgeon lately returned from the Afghan War, needs a flat-mate and a diversion.

Sherlock Holmes needs a foil. And thus a great literary collaboration begins. Then murder. Facing a deserted house, a twisted corpse with no wounds, a mysterious phrase drawn in blood on the wall, and the buffoons of Scotland Yard--Lestrade and Gregson--Holmes measures, observes, picks up a pinch of this and a pinch of that, and generally baffles his faithful Watson.

Later, Holmes explains: "In solving a problem of this sort, the grand thing is to be able to reason backward...There are few people who, if you told them a result, would be able to evolve from their own inner consciousness what the steps were which led up to that result." Sherlock Holmes is one of those people.