Tufnell was not aware that in the former village boy who had become a London detective he was in the presence of a young man of soaring ambition. Caldew had gone to London fifteen years before with the idea of bettering himself. After tramping the streets of the metropolis for some months in a vain quest for work, he had enlisted in the metropolitan police force rather than return to his native village and report himself a failure. At the end of two years' service as a policeman he had been given the choice of transfer to the Criminal Investigation Department of Scotland Yard. He had gladly accepted the opportunity, and had shown so much aptitude for plain-clothes work that by the end of another two years he had risen to the rank of detective. Caldew thought he was on the rapid road to further promotion, and had married on the strength of that belief. But another ten years had passed since then, and he still occupied a subordinate position, with not much hope of promotion unless luck came his way. And there seemed very little chance of that. Caldew's professional experience had imbued him with the belief that the junior officers of Scotland Yard existed for no other purpose than to shoulder the blame for the mistakes of their official superiors, who divided amongst themselves the plums of promotion, rewards, and newspaper publicity. That, of course, was the recognized thing in all public departments. Caldew found no fault with the system. His great ambition was to obtain some opening which would bring him advancement and his share of the plums. He believed his opportunity had arrived that night. It had always been his dream to have the chance to unravel single-handed some great crime--a murder for choice--in which he alone should have all the glory and praise and newspaper paragraphs. He determined to make the most of the lucky chance which had fallen into his hands, before anybody else could arrive on the scene. He had confidence in his own abilities, and thought he had all the qualifications necessary to make a great detective. He was, at all events, sufficiently acute to realize that opportunity seldom knocks twice at any man's door.