This story began when one of the young assistants of Sherlock Holmes saw at the London market how a strange old woman accosted greengrocers, asking about the exotic fruit pitahaya. What could possibly connect this strange fact with the mysterious death of a provincial nobleman and a small businessman, Edmund Quallsford? To find some link between these facts, Holmes will have to travel to the so-called country of swamps in the south of England. And his new assistant, Jones, will help him in the investigation.