The intended wedding of Captain Hugh "Bulldog" Drummond to Phyllis Clavering at her villa in Switzerland is once again stopped short when someone murders the Swiss policeman who is guarding their wedding presents. The killer makes off with their prize possession, a synthetic diamond, made by a secret process by Professor Bernard Goodman, the father of their good friend Gwen Longworth. A guest, Sir Raymond Blantyre, head of the Metropolitan Diamond Syndicate, disappears at the same time, and Drummond suspects that Sir Raymond, who has the most to lose if Professor Goodman proceeds with his plans to publish his secret process, has something to do with the theft. He leaves Phyllis and returns to England.
Colonel Nielsen of Scotland Yard scoffs, as usual, at Drummond's suspicions and insists that a man as respected as Sir Raymond could not possibly be involved in such a crime. An explosion that wrecks Goodman's house, and apparently kills him, makes Drummond more positive that the diamond king has again resorted to murder to protect his business. He follows Professor Botulian, a lifelong rival of Goodman's, whom he believes to be involved in the affair. His hunt leads him to a lonely house on the outskirts of London where he finds Goodman a prisoner. Drummond's valet Tenny soon joins them as captive, but brings with him the means of escape. After Goodman is taken to safety, Drummond discovers that Phyllis, who was searching for him, is now being held by the crooks. Drummond quickly returns to the house to confront Sir Raymond and his armed confederates. Drummond begins to fight his way out, but is met by superior forces.
