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On the eve of British detective Hugh Drummond's fifth attempt to wed Phyllis Clavering, one of his house guests, Professor Downie, is murdered after he discovers the location of a treasure buried during the reign of King Charles in a subterranean cavern of Rockingham Tower. When Phyllis is mysteriously spirited away by murderer Borjei Islanyani, who has been masquerading as her butler Albert Boulton, Drummond, his butler, Tenny, and his guests, Algy Longworth and Colonel Nielson of Scotland Yard, descend into the honeycombed tunnels of Drummond's estate to rescue her. The tunnels are riddled with medieval traps worthy of a chamber of horrors, and in one of these, Borjei imprisons the detectives by lowering a set of iron bars. As vicious spikes descend to crush the detectives, Phyllis frees her rescuers by reversing the wheels of the ancient contraption. In the ensuing struggle, the flood gates open and sweep Borjei into the surging waters of the Thames River. The murderer thus disposed of, Phyllis and "Bulldog Drummond's secret police" then climb to safety and are about to resume the wedding festivities when the castle ceiling collapses and Phyllis beats a hasty exit.