In The Case of the Black Gauntlet, Thea Hertz was a German stunt pilot and actress who had been contracted to act in a film being produced by Group Captain "Wizard" Gainsforth. When she had been hired, she had insisted that an expert direct the air combat sequences and suggested Biggles. In view of her popularity and her importance to the film, Gainsforth agreed, and began to scheme how to bring Biggles on board, not knowing what Thea's real motives were.
Only when Biggles and Thea were airborne for the air sequence did Thea call Biggles over the radio to reveal her true intentions. Apparently Biggles had shot down and killed her brother over Marham during World War 2. Thea had witnessed the scene as she was there working for the U.S. 8th Air Force (she was actually a German spy). Now Thea had live ammunition loaded into her aircraft. She intended to kill Biggles and then drop incendiaries to kill the film crew on the ground. After that, she intended to defect to East Germany.
Biggles had a dilemma as he felt he couldn't shoot at a woman even assuming he had real ammunition in the first place. At the same time, he had to thwart Thea's threat to bomb the film crew. In the ensuing air combat, Thea's lack of experience became apparent and time and again, Biggles got the better of her--Biggles hoping this would bring her to her senses. However Thea pressed her attacks and eventually Biggles in evading her, went low, flying under some telegraph wires which he saw just in time. Thea followed but failing to see the wires, hit them and was killed in the resultant crash.
Much later, after the film had been released, Gainsforth told Biggles that he had seen Thea load live ammunition into her guns. Alarmed at this, he quietly had them replaced with blanks but refrained from telling Biggles so that the flying manoeuvres for the film would be more realistic and exciting.