Out of these dreams he was awakened by a sound that had slowly and persistently become a part of his mental consciousness.

It was a tap, tap, tap at his window.

At last he sat up and listened.

It was in the gray gloom of dawn.

Again the sound was repeated: tap, tap, tap on the pane of glass.
He slipped out of bed, his hand seeking the automatic under his pillow.
He had slept with the window partly open.