Andrew Lauriston, a young writer, visits the premises of a pawnbroker only to find the man's dead body in suspicious circumstances. At first he is the prime suspect but is soon cleared of suspicion through the actions of his friends, John Purdie and Melky Rubenstein, the grandson of the pawnbroker. They decide to investigate the pawnbroker's death and are led on a chase across London in search of a very valuable orange-yellow diamond. They become involved with an MP, an American, a Japanese doctor and two Chinese medical students before the mystery is solved.