"Very well! But first I want to show you something.

Will you come here, please, Mr.Anderson ?" She started for the alcove.
"I've examined that staircase," said the detective.
"Not with me!" insisted Miss Cornelia.

"I have something to show you." He followed her unwillingly up the stairs, his whole manner seeming to betray a complete lack of confidence in the theories of all amateur sleuths in general and spinster detectives of sixty-five in particular.
Their footsteps died away up the alcove stairs.

The living-room was left vacant for an instant.
Vacant?
Only in seeming.

The moment that Miss Cornelia and the detective had passed up the stairs, the crouching, mysterious Unknown, behind the settee, began to move.