In The Unknown Diamonds, Leffers was a pilot who worked for Dr. Shultz, a research scientist working in the deserts of Southwest Africa (now Namibia), collecting monkeys for sale to zoos and research laboratories around the world. Leffers flew the monkeys in an old converted wartime Dornier modified to transported 60 animals at a time. The Dornier flew to Algiers where the monkeys were then transferred a depot there and later shipped to various locations by regular air services.
As a sideline, Dr. Shultz also began smuggling diamonds. These were brought to him by the Kalahari bushmen, who regularly found them in the desert gravel. Shultz traded these diamonds for cheap trinkets such as pipes and jackets and also supplied them with tobacco and jerry cans of water. Shultz then had these diamonds surgically implanted into his monkeys which were then sent by air to his brother, also named Shultz in Britain where the diamonds, smuggled in free of duty, were sold and the proceeds donated to a charitable foundation in Germany caring for disabled soldiers.
Biggles visited Dr. Shultz's establishment while investigating the appearance of unknown diamonds which had been found at a pawnbroker's shop in London. Dr. Shultz showed him a batch of monkeys which Leffers was about to ship to Europe. Biggles, examining the animals, discovered lumps in the neck of one with a grey face. Thinking he had uncovered the details of the scheme, Biggles flew to Algiers, arriving before Leffers, and asked Marcel to maintain surveillance on Leffer's cargo. Marcel learnt that most of the monkeys would be staying at a depot in Algiers but the grey-faced one was consigned to London. Biggles then flew on to London and arranged for the grey-faced monkey, to be intercepted at the port of entry at London Airport where Shultz, the doctor's brother was arrested and confessed to the whole scheme.
Dr. Shultz and Leffers were never seen again. They had fled by the time the Southwest African police reached his establishment. It was assumed that he and Leffers flew to Germany in the Dornier, either because they took fright after Biggles' visit or they had received a warning message from Shultz in London.