A jewellery theft is reported by Lord Langdon. Biggles heads to Africa to track down the missing Ruby collection. The stones are believed stolen by the missing manservant, although the daughter of the house seems to know a little more about the theft than she should.

Biggles and Bertie track the gentleman to a deserted German fort in the Kalahari. After rescueing him from a mad Irishman he explains that the jewels were not stolen as much as hidden. He turns out to forgotten son of the Lord who reported the theft, and was hiding the stones to protect the inheritance of his half-sister.

Biggles and Bertie return to England to explain. At the end they receive notice that the prodigal son has returned to the house and all is well with the Langdons.