Chapter 20: Adelaide Exposed — The Exchange Under Surveillance

The knock came at seven minutes past seven, and Watson knew before he reached the door.

Not because there was anything distinctive in the sound itself — three knocks, measured, not panicked — but because no one who had legitimate business with him arrived at seven in the morning without prior arrangement, and the people who might arrive without prior arrangement did not knock like that. They knocked like Perkins, which was to say with the particular rhythm of someone who had run most of the way and was trying not to show it. Or they knocked like Lestrade, which was to say with the authority of a man who was accustomed to doors opening when he willed them to.

This knock was neither. This knock was controlled in the way that a person controls themselves when the alternative is coming apart in a stranger's doorway.