Professor Moriarty commits a small fraud to expose a larger one. A young stockbroker brings a dilemma to the Moriartys which can best be solved by a quiet substitution. Professor Moriarty will turn up for the new position at the dubious investment bank, while the stockbroker spends his days in Russell Square. Moriarty uncovers a dastardly scheme, but the stockbroker's wife, alarmed by her husband's altered behavior, hires Sherlock Holmes to investigate. The two rivals confront one another in the midst of a carefully planned caper.