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Assistant-Commissioner Air Commodore Raymond asks Biggles to investigate the theft of a collection of rubies from Lord Langdon. Langdon suspects the thief is his former footman, Richard Browning, who has disappeared after being warned for being too familiar with Langdon's daughter, Caroline. Interviewing Lady Caroline gets Biggles nowhere as she is clearly protecting Browning. Biggles discovers that Browning is writing to Lady Caroline via her former nanny, Mrs. Smith and the letters are coming from Africa. The stamp on the letter is smudged but four letters "W-I-N-D" can be made out. Clues lead Biggles to suspect the letter is from Windhoek on the edge of the Kalahari Desert in South West Africa. Biggles and Bertie fly to Africa in an attempt to find Browning. Searching the Kalahari, they are shot at and this leads them to land at an old abandoned fort, called Fort Schwarz. Here they find a chained up leopard (the picture on the dust cover, together with a picture of (presumably) Biggles, a spear and a bullet). They also meet Mick Connor, Browning's former partner who says he knows nothing of Browning's whereabouts and is most unhelpful. Returning to their plane they have a confrontation with a wounded rhinoceros. That night Biggles goes to investigate the fort under the cover of darkness, and a native shoots at him with a bow and poisoned arrow. The following morning, Connor leaves the fort giving Biggles and Bertie a chance to search it thoroughly. Here they find Browning, injured after a plane crash and he tells them his story. Browning is in fact, the long lost son of Lord Langdon who got the job as footman to find out what his father was like. Browning says the rubies belong to his sister Lady Caroline and he only took them to hide them so that Lord Langdon couldn't sell them off. Lady Caroline knows their whereabouts, he says. Connor returns, having only left the fort in order to trap Biggles and Bertie. He cuts up rough but is killed by a native with a bow and poisoned arrow because he has previously murdered a native. Biggles returns to England to confront Lord Langdon with the result of his enquiries and enable the family to bury the hatchet.