Inspector McLean has resigned from Scotland Yard's CID and has opened an office in Wimpole Street, where there is a small gilt owl on a bracket over his door and a brass plate inscribed "Robert McLean, Private Investigator". With the help of his able assistant, Tiny, he solves many crimes, including cases of kidnapping, murder and poisoning. He also investigates the conspiracy against him which led to his resignation from the Yard, and seeks revenge against the man who framed him.