"Whatta fool!" roared the big Italian. and raising the revolver he fired point-blank at the woman's retreating figure just as I flung myself upon him.

But the weapon merely clicked harmlessly and Poirot's voice rose in mild reproof.

"Never will you trust your old friend, Hastings. I do not care for my friends to carry loaded pistols about with them and never would I permit a mere acquaintance to do so. No, no, mon ami." This to the Italian who was swearing hoarsely. Poirot continued to address him in a tone of mild reproof:

"See now, what I have done for you. I have saved you from being hanged. And do not think that our beautiful lady will escape. No, no, the house is watched, back and front. Straight into the arms of the police they will go. Is not that a beautiful and consoling thought? Yes, you may leave the room now. But be careful - be very careful. I - Ah, he is gone! And my friend Hastings looks at me with eyes of reproach. But it was all so simple! It was clear, from the first, that out of several hundred, probably, applicants for No. 4, Montagu Mansions only the Robinsons were considered suitable. Why? What was there that singled them out from the rest - at practically a glance? Their appearance? Possibly, but it was not so unusual. Their name, then!"

"But there's nothing unusual about the name of Robinson," I cried. "It's quite a common name."

"Ah! Sapristi, but exactly! That was the point. Elsa Hardt and her husband, or brother or whatever he really is, come from New York, and take a flat in the name of Mr and Mrs Robinson. Suddenly they learn that one of these secret societies, the Mafia, or the Camorra, to which doubtless Luigi Valdarno belonged, is on their track. What do they do! They hit on a scheme of transparent simplicity. Evidently they knew that their pursuers were not personally acquainted with either of them. What then can be simpler? They offer the flat at an absurdly low rental. Of the thousands of young couples in London looking for flats, there cannot fail to be several Robinsons. It is only a matter of waiting. If you will look at the name of Robinson in the telephone directory, you will realize that a fair-haired Mrs Robinson was pretty sure to come along sooner or later. Then what will happen? The avenger arrives. He knows the name, he knows the address. He strikes! All is over, vengeance is satisfied, and Miss Elsa Hardt has escaped by the skin of her teeth once more. By the way, Hastings, you must present me to the real Mrs Robinson - that delightful and truthful creature! What will they think when they find their flat has been broken into! We must hurry back. Ah, that sounds like Japp and his friends arriving."

A mighty tattoo sounded on the knocker.

"How did you know this address?" I asked as I followed Poirot out into the hall. "Oh, of course, you had the first Mrs Robinson followed when she left the other flat."