John Mannering (aka "The Baron") is a former cracksman and jewel thief who is occasionally consulted by Scotland Yard when there is a particularly difficult, or mysterious, case to solve. Seemingly reluctantly, Superintendent Bristow contacts him because the police need help in tracking down some stolen precious stones, which have so far eluded them. Mannering suspects some sort of elaborate trap is being set and so very carefully lays down his own terms before rendering assistance. Then things change dramatically as the crime being investigated escalates from theft to murder. Were Mannering's instincts correct in the first place?