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.

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.

His hunt leads him to a lonely house on the outskirts of London where he finds Goodman a prisoner.