In Sergeant Bigglesworth C.I.D., Ludwig Renkell was a German aircraft designer who had developed a number of innovative prototypes just as the Second World War was coming to a close. These included a twin-engine long range fighter named the Wolf and a light bomber, both said to have exceptional performance. Anticipating the end of the war, he had also begun developing a civilian light aircraft named the Swan. After the war ended, the British authorities discovered that Renkell's prototype combat aircraft had disappeared from his aircraft works at Augsburg. Renkell and his chief test pilot Kurt Baumer were also nowhere to be found. Staff at the works insisted that they knew nothing about the aircraft.
Investigations by Biggles later discovered that the aircraft had been taken by a gang of jewellery robbers led by ex-Nazi Julius Hans Gontermann and his friend, Renkell's test pilot Kurt Baumer. Renkell had been kidnapped by the gang and used as a mechanic for the machines. Renkell's works manager Rudolf Preuss was also in on the gang and used the Swan as a communications aircraft between the gang in North Africa and his base at Augsburg.
Renkell was kept at the gang's main base which was located in a belt of sanseviera forest just inside the Abyssinian border opposite Sudan. After Biggles destroyed the base, the gang members departed in the prototype aircraft, allowing Renkell to make his escape. He was discovered by Algy and Ginger and brought back to civilisation where his emaciated and frail state lent credence to his claim that he was a prisoner and not a willing member of the gang.