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Ten years after the unsolved murder of Captain Eli Wetherby, his grandniece Elizabeth "Betty" Woods searches his boat, the Black Joker , for hidden treasure with her fiancé, David Courtland. Betty's belief in the existence of the treasure comes from listening to The Man Who Lifts the Veil , a radio show featuring amateur detective H. H. Van Buren. Van has been dramatizing her uncle's case and has promised his listeners that he will solve the crime during next week's show. While Betty leaves the boat, which is docked in New Jersey, to return to her New York City apartment, Van is confronted by New Jersey police inspector Norris at the radio station. Norris threatens to arrest Van for obstructing justice if he does not reveal Wetherby's killer, but when Van tells him that he suspects a man known as Manuel Dazetta, Norris laughingly tells him that Wetherby's alias was Dazetta. After Norris leaves, Van's sponsor, Conroy, orders him to go to the Black Joker and uncover the real killer. Van departs with his black valet, Euclid Brown, but his arrival at the boat is preceded by that of two actors, Stella and Norbert, who have been hired by Van's announcer, Jerry Gilpin. Van and Gilpin are engaged in an ongoing and escalating war of practical jokes, and Gilpin has hired Stella and Norbert to enact the roles of Wetherby's crazed fiancée Meg, and Wetherby's first mate, Long Jack. Their frightful act succeeds in terrifying Euclid, but Van overhears them discussing their roles and deduces the truth. When Betty appears, however, Van thinks that she is also an actress and does not believe her when she says that her uncle's lawyer gave her two clues to finding the treasure, a dog collar and a will, stating that she must take the same trip that Wetherby took on 23 Sep 1929. As Van is searching the boat, Professor Walter Bascomb, an oceanographer, and traveling book salesman Jonathan Flack board, stating that they became lost while driving in the heavy fog. Norris also arrives, and much confusion ensues for Van as the dog collar is stolen and a dead sailor, Mack Wolf, is found. Wolf was one of Wetherby's men, and Van eventually realizes that Bascomb is another of the former crew, then known as Jackson Voker. Bascomb is attacked, but before collapsing, he tells Van to check Wetherby's logbook. Van and Euclid use a large globe to plot out Wetherby's listed course on 23 Sep 1929, which turns out to be the combination that opens a secret compartment in the globe. Inside is what appears to be a fortune in diamonds, and Bascomb then reveals that although he did kill Wolf, it was Norris, whose real name is Manuel Dazetta, who killed Wetherby in order to get the diamonds. Norris has been searching for the stones ever since Wetherby found them and is bitterly disappointed when Jonathan states that they are merely Mexican rock crystals. Soon everyone is safely off the boat, and Van and Euclid travel back to New York City with Norris as their prisoner.