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Harry Sanders loses his job as a conservation officer in Africa and is hired by a London insurance company to investigate the loss of a diamond dredge owned by wealthy Texan A. J. Magnus, who is heavily insured. Magnus buys a new dredge, and Sanders, assisted by Tom Hamilton, boards the vessel and meets its German captain, von Koltz, an ex--submarine commander. Sanders also meets the captain's wife, Elizabeth (who is having an affair with her husband's boss, Magnus), and his sister, Helga, with whom Sanders falls in love. Sanders quarrels with Carlo Seton, one of Magnus' henchmen, while von Koltz anchors the dredge and explores underwater. His curiosity aroused, Sanders follows von Koltz underwater and is instrumental in saving his life. After the rescue, von Koltz confides to Sanders his plan to find a secret gold consignment that was carried on a ship he sank in wartime. With the assistance of Charlie Singer, Sanders discovers that Magnus' lost dredge is stranded on the sandy shore of the "Coast of Skeletons." Sanders also discovers that the diamond dredging is a front to hide the fact that the wreck contains the gold that von Koltz seeks. Magnus, who, along with Elizabeth and Carlo, has been planning to doublecross von Koltz and Singer, finds the gold, and Carlo shoots Singer. Before dying, Singer ignites a cache of dynamite, and Magnus and his companions die in the explosion. Sanders, Helga, Hamilton, and von Koltz witness the explosion and escape from the danger, planning to return later to recover the gold.