Upon receiving a mysterious message from his old friend, Richard Gannett, English detective Hugh Drummond, stops by Gannett's flat while on his way to his bachelor dinner, and finds his friend mortally wounded. Once the police and Hugh leave, the murderer, Rolf Alferson, returns to dismantle Gannett's invention, a ray that will detonate explosives from half a mile away. Soon after, Hugh, accompanied by his friend, Algy Longworth, returns to the scene of the crime and, upon finding another corpse, decides to investigate for himself. When Hugh receives a warning that unless he gives up his investigation, someone close to him will be harmed, he sends his fiancée Phyllis Clavering and her aunt Meg on what was to be their honeymoon cruise. Next, Hugh, his butler, Tenny, and Algy trace the machine to the apartment of Gannett's neighbor, Lady Beryl Ledyard, just in time to see Beryl and Alferson leaving with the ray. Too late, Hugh learns that Beryl and Alferson have sailed on the same cruise as Phyllis, and after being empowered to investigate the crime by Colonel Nielson of Scotland Yard, Hugh, Tenny and Algy fly to meet the ship on St. Anthony's Island. Once ashore, Hugh meets Phyllis and sends Tenny and Algy to pose as tourists to see if they can locate Alferson's hideout. Upon discovering their true identity, Alferson captures them and uses Algy's ring to set a trap for Hugh. However, Phyllis intercepts the ring and goes in Hugh's place, but Algy and Tenny escape in time to warn Hugh, who captures Alferson and Beryl and saves Phyllis.
