A bid for fortune or Dr Nikola's vendetta

This is the first in Boothby's series of Dr. Nikola novels. It opens with the manager of a fine London restaurant preparing for a dinner reserved in an odd letter originating from Brazil 3 months prior, for 4 persons in a private room, where 3 guests will arrive showing cards with a red dot. One will be from China, one from South Africa, one from England, plus the host, Dr Nikola, who is of unknown nationality but is later revealed to travel the world widely. The letter specifies certain preparations, such as no electric lighting but rather candles with red shades, a small jug of fresh milk, and a porcelain saucer. After the 3 guests arrive, Dr Nikola makes his appearance. He is a somewhat tall slim man, "irreproachably dressed, with a neatness that bordered on the puritanical." After they dine and the servants leave the room, Dr Nicola opens a strange basket and removes a mysterious black cat, for whom the milk had of course been requested. Dr Nikola explains why he called them together -- to execute his plans of revenge against a very wealthy someone who did him great harm. He offers each of them a large sum of money if they perform as he asks: "I demand from you your whole and entire labour. While you are serving me you are mine body and soul. I know you are trustworthy. I have had good proof that you are--pardon the expression--unscrupulous, and I flatter myself you are silent. What is more, I shall tell you nothing beyond what is necessary for the carrying out of my scheme, so that you could not betray me if you would. Now for my plans!"