The Clue of the Silver Key by Edgar Wallace

JULES BARELY DESERVES description because he plays so small a part; but as that small part was big enough to put one man in the shadow of the gallows, he may be catalogued as a plump, sallow-faced young man with perfectly brushed hair, who was invariably dressed as though he were on his way to a wedding reception.

He was a sort of attache to a South American Legation, and a free-lance of diplomacy generally. In more suspicious countries he would have been handed his passport with extreme politeness, and his departure from Southampton would have been watched by the bored detective whose business it is to superintend the shipment of oddities.