Now that he is retired as a master 'cracksman', John Mannering (aka 'The Baron') is sometimes consulted by Scotland Yard when they have a particularly difficult, or unusual, case. When a jeweller who himself walks the fine line between acceptable and unlawful behaviour is robbed, the Baron's opinion is sought. However, he is on the trail with another case involving missing diamonds. Then he discovers that not only are the two crimes connected, but he is himself the chief suspect.