The 15th book goes back to 1907
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The story begins in summer, 1907, ten years after the Emersons' expedition into the Nubian desert in The Last Camel Died at Noon, when the Emersons were lured to a Lost Oasis where the remains of a Meroitic - Ancient Egyptian civilization that had avoided the outside world for centuries still survived. It was during that journey that the Emersons brought back Nefret Forth to live with them in England. A messenger from the Lost Oasis appears at their home in Kent, pleading for help for their friend, King Tarek, and they have no choice but to go to his aid.