The death of Professor Goodman is officially recorded as a tragic accident, but at the inquest, no mention is made of his latest discovery - a miraculous new formula for manufacturing flawless diamonds at negligible cost, which strikes Captain Hugh 'Bulldog' Drummond as rather strange. His suspicions are further aroused when he spots a member of the Metropolitan Diamond Syndicate at the inquest. Gradually, he untangles a sinister plot of greed and murder, which climaxes in a dramatic motorboat chase at Cowes and brings him face to face with his archenemy.