London was drowning in fog. The evening had settled heavily over the city, wrapping the narrow streets in a veil of mist that blurred the streetlamps into pale golden halos. Rain had fallen earlier, leaving the pavement slick and shining beneath passing lights. Most people hurried by with lowered heads, eager to escape the damp cold.

At first, the black dog seemed unremarkable. It stood at the far end of the street, half-hidden by the fog, watching. Large and shaggy, it looked like a stray that had wandered in from somewhere forgotten. Yet there was something unusual in the way it stared—not with the vacant curiosity of an animal, but with careful attention, as though it were trying to recognize a face after many years.

When you continued walking, the dog followed, but not closely. It lingered at a distance, padding silently through the mist whenever you moved, stopping whenever you stopped. More than once it seemed about to approach, only to hesitate and remain where it was, standing beneath a lamp or beside a dark doorway.

Eventually it started to get annoying.