An insurance company hurt by a string of jewel thefts decides to smoke out the international community of thieves by putting the priceless Karenina diamond necklace up for auction. There are many interested parties, but jewel dealer Pierre Londais ultimately wins the bidding. Among the suspicious characters interested in the diamonds are a young woman named Odette Mauclair, a wealthy American named Colonel Jackson, and the mysterious Baron Von Kampf. There are robbery attempts at Londais's Paris hotel and later aboard the train to Istanbul. When the diamonds go missing, everyone aboard the train is a suspect.