Thomas Avery vanished from a boarding house without taking his coat, boots or money.

His room was locked.

The window was open.

The landlady heard no disturbance.

Cryer inspected the room.

Everything was orderly.

A half-written letter lay upon the desk.

It ended:

"I have discovered where the sound is coming from."

Nothing followed.

Cryer searched the building.

Behind the wallpaper of the bedroom he discovered a narrow vertical cavity in the wall.

It contained an old ventilation shaft leading downward.

The shaft was too narrow for a man.

At the bottom, however, Cryer found Avery's pocket watch.

The watch was still running.

Cryer descended into the cellar.

There he heard something.

A faint tapping.

Three knocks.

He followed the sound to a brick wall.

The tapping stopped.

No cavity was found behind the bricks.

Avery was never located.

His belongings remained in the room for six months.

Then, one morning, the unfinished letter was gone.