My name is Charles Wilson. I was twenty-five that June of 1929. Although I had lived all of my adult life in New York City, I had no relatives there and few friends. * * * * * I had known Captain Mason for several years. Like myself, he seemed one who walked alone in life. He was an English gentleman, perhaps thirty years old. He had been stationed in the Bermudas, I understood, though he seldom spoke of it. I always felt that I had never seen so attractive a figure of a man as this Derek Mason. An English aristocrat, he was, straight and tall and dark, and rather rakish, with a military swagger. He affected a small, black mustache. A handsome, debonair fellow, with an easy grace of manner: a modern d'Artagnan. In an earlier, less civilized age, he would have been expert with sword and stick, I could not doubt. A man who could capture the hearts of women with a look. He had always been to me a romantic figure, and a mystery that seemed to shroud him made him no less so. A friendship had sprung up between Derek Mason and me, perhaps because we were such opposite types! I am an American, of medium height, and medium build. Ruddy, with sandy hair. Derek Mason was as meticulous of his clothes, his swagger uniforms, as the most perfect Beau Brummel. Not so myself. I am careless of dress and speech. I had not seen Derek Mason for at least a month when, one June afternoon, a note came from him. I went to his apartment at eight o'clock the same evening. Even about his home there seemed a mystery. He lived alone with one man servant. He had taken quarters in a high-class bachelor apartment building near lower Fifth Avenue, at the edge of Greenwich Village. All of which no doubt was rational enough, but in this building he had chosen the lower apartment at the ground-floor level. It adjoined the cellar. It was built for the janitor, but Derek had taken it and fixed it up in luxurious fashion. Near it, in a corner of the cellar, he had boarded off a square space into a room. I understood vaguely that it was a chemical laboratory. He had never discussed it, nor had I ever been shown inside it. Unusual, mysterious enough, and that a captain of the British military should be an experimental scientist was even more unusual. Yet I had always believed that for a year or two Derek had been engaged in some sort of chemical or physical experiment. With all his military swagger he had the precise, careful mode of thought characteristic of the man of scientific mind. * * * * * I recall that when I got his note with its few sentences bidding me come to see him, I had a premonition that it marked the beginning of something strange. As though the portals of a mystery were opening to me!