"It is a trick of the light, Watson," Holmes remarked, his back to the window. He was furiously scrubbing a test tube. "A combination of coal smoke, a slight imperfection in the Victorian glass, and the overactive imagination of a public desperate for the divine."

Sherlock Holmes did not believe in ghosts, but the ghost of 221B Baker Street believed in Sherlock Holmes .

"Identity?" Holmes whispered, his hand hovering over his magnifying glass.

"I do not believe in you," Holmes said, though his eyes were wide.

She vanished. The room warmed instantly. On the floor, where she had stood, lay a single, very real scrap of paper. Holmes picked it up with trembling fingers. It wasn't a clue for a murder or a heist. It was a name and an address of a woman in East End whose son had gone missing—a case Holmes had dismissed as a "common runaway" only that morning.

But then came the fourth night. The temperature in the sitting room plummeted. The fire in the hearth turned a sickly, chemical green. Holmes finally turned.