“Ah!  I hoped that would be so.  Can you ascertain for certain?”

“But why?”

“Because I wish to go in there.  And that brings me to the favor I seek.  The secretary of these flats, even the hall porter, should have a master key.  Borrow it on some pretext.  They will give it to you.”

“Really, Mr. Forbes—­” gasped Theydon, voicing his surprise as a preliminary to a decided refusal.  He was interrupted by the insistent clang of the telephone—­ that curt herald which brooks no delay in answering its demand for an audience.

“Pardon me one moment,” be said.  “I’ll just see who that is.”

The inquirer was Evelyn Forbes.

“I’ve waited patiently—­” she began, but he stopped her instantly by saying that her father was with him.

“Please ask him to come to the phone,” she said.

Forbes rose at once.  He merely assured the girl that he was engaged in important business and would be home soon after the luncheon hour.  Meanwhile, she was not to go out, and his orders must be obeyed to the letter.

“Now, Theydon,” he said, coming back to the sitting room, “what about that key?”

The most extraordinary feature of an extraordinary case was the way in which the mere sound of Evelyn Forbes’s voice stilled any qualms of conscience in Theydon’s breast.  He knew he was acting foolishly in conducting a blind inquiry on his own account, an inquiry which might well arouse the anger and active resentment of the police, but he offered a sop to his better judgment by consulting Bates.

Then came a veritable surprise.

“The fact is, sir,” admitted Bates nervously, “we have Ann Rogers’s key in the kitchen.  When she went away on Monday she left it here, bein’ afraid of losin’ it.  Of course, she took it on Tuesday mornin’, and after goin’ from one fit of hysterics into another she gev it to us again.”

Theydon’s face was eloquent of the serious view of this avowal.

“Did you tell the police?” he said.

“No, sir.  My missus an’ me clean forgot all about it.”

“So, while Mrs. Lester was being killed, the key of her flat was actually in your possession?”

“I suppose it might be put that way, sir.”

By this time Theydon was becoming exasperated at the veritable conspiracy which fate had engineered for the express purpose, apparently, of entangling him in an abominable crime.

“Why on earth didn’t you mention such an important fact to the detectives?” he almost shouted, “Don’t you see they are bound to think—­”

“O, a plague on the detectives and on what they think!” broke in Forbes imperiously.  “It doesn’t matter a straw what they think, and very little what they do.  This affair goes a long way beyond the four-mile radius, Theydon.  The vital point is that your man has the key.  Where is it?  Let us go in there at once!”

“You offered me some advice, Mr. Forbes,” said Theydon firmly.  “Let me now return it in kind.  If you wish to examine Mrs. Lester’s flat why not seek the permission of Scotland Yard?”