In Biggles Works It Out, Wilhem Groot was a German pilot of and operative of Count Heinrich Horndorf's criminal gang known as "The White Prophets of Peace". Groot took part in ambush and robbery of the gold convoy at Sandy Bottoms where he lost a button from his jacket (which Scotland Yard experts determined must have been green or grey) and dropped the sleeve of a toothpick which he had obtained from the Hotel de Paris at Monte Carlo. When Biggles, with the assistance of the Australian aborigine tracker Joe, found these items at the crime scene, he sent Algy and Bertie to the hotel at Monte Carlo to watch for an individual who had lost a jacket button from a grey green jacket and who picked his teeth.
Although the chances of success were very slim, Algy encountered just such an individual at the Hotel de Paris after five days on the job. Algy followed the man and entered into his bathing cabin while he was at the beach to search his clothes. There Algy found that his name was Wilhelm Groot and that he had an address at the Villa Hirondelle in the nearby town of Eze, which turned out to be the residence of a wealthy Austrian Count Heinrich Horndorf.
After delivering his report to Biggles in London, Algy was sent back to watch the Villa again. He followed a van carrying von Stalhein and Groot from the villa to an disused airstrip in the Plaine de la Crau which turned out to be a landing ground for the gang's aircraft coming in from North Africa. There, Von Stalhein and Groot met a Douglas DC-3 piloted by Dick Canton and Luis Leguez which had just flown in from El Asile. As Algy stole onboard the DC-3 which Canton wanted to take back to El Asile, the surviellance of Groot was broken off.
Towards the end of the book, Groot reappeared when he piloted von Stalhein to El Asile in a Mosquito. There he got caught in a raid of the oasis by Tuareg tribesman and made a hasty escape. He landed the Mosquito at the Plaine de la Crau and was ambushed by French gendarmes while stepping off the aircraft. Groot tried to make it back on board on board but Von Stalhein, who was still in the cockpit, abandoned him and took off. Groot resisted arrest and was shot dead in the subsequent gun fight.