“Do you mean detectives from Scotland Yard?” he cried.

“Yes.”

Forbes smiled, and commenced filling a pipe.

“Evidently they did not want you as a principal,” he said.

His tone was genial, but slightly guarded.  Theydon realized that this man of great wealth and high social position had reminded himself that his guest, though armed with the best of credentials, was quite unknown to him otherwise, and that, perhaps, he had acted unwisely in inviting a stranger to his house without making some preliminary inquiry.  This reversal of their roles was a conceit so ludicrous that Theydon smiled too.

At any rate, he meant now to pursue an unpleasing task, and have done with it.

“No,” he said slowly.  “It seems that I am the worst sort of witness in a murder case.  I may have heard, I may even have seen, the person suspected of committing the crime, or, if that is going too far, the person whom the police have good reason to regard as the last who saw the poor victim alive and in ordinary conditions.  But my testimony, such as it is, is so slight and inconclusive that, of itself, no one could hang a cat on it.”

“Good gracious!  That sounds interesting, though you have my sympathy.  It must be rather distressing to be mixed up in such an affair, even indirectly.”

Forbes struck precisely the right note of friendly inquiry.  He wished to hear more, and was at the same time relieved to find that Professor Scarth had not introduced a notorious malefactor in the guise of a young writer seeking material for an article on air-ships!

Theydon could have laughed aloud at this comedy of errors, but the fact that at any moment it might develop into a tragedy exercises a wholesome restraint.

“I happen to live at No. 18 Innesmore Mansions,” he said.  “Opposite—­ on the same floor, I mean—­ lives, or did live, a Mrs. Lester.  I do not—­”

“Are you telling me that a Mrs. Lester of No. 17 Innesmore Mansions is dead—­ has been murdered?”

Forbes’s voice rang out vibrant, incisive.  His ordinarily pale face had blanched, and his deep-set eyes blazed with the fire of some fierce emotion, but, beyond the slight elevation of tone and the change of expression, he revealed to Theydon’s quietly watchful scrutiny no sign of the terror or distress which an evildoer might be expected to show on learning that the law’s vengeance was already shadowing him, even in so remote a way as was indicated by the presence under his roof of a witness regarded by the police as an important one.

“Yes!” stammered Theydon, quite taken aback by his companion’s vehemence.  “Do you—­ know the lady?  If so—­ I am sorry—­ I spoke so unguardedly—­”

“Good heavens, man, don’t apologize for that!  I am not a child or weakling, that I should flinch in horror from one of life’s dramatic surprises!  But, are you sure of what you are saying?  Mrs. Lester murdered!  When?”