In Biggles Takes a Hand, Max Lowenhardt was a friend of Oberst Hans Roth and Dr Bruno Jacobs. His daughter, Anna, was engaged to Moritz Roth, son of Hans Roth. Two years before the events in the book, Anna had also worked in London and had stayed with Dr Jacobs.
Max Lowenhardt was Professor of European History at a university in West Berlin and lived at Antonstrasse, No 71 with his daughter Anna and their housekeeper Gretchen. When the family of Hans Roth escaped to West Berlin after the arrest and execution of Hans Roth, Max Lowenhardt gave them refuge in his house. He and Anna then sent them on to London, with a letter of introduction to Dr Bruno Jacobs, believing that they would be safe there.
When Biggles called at Antonstrasse hoping to find some way of tracing the Roths in London, he failed to find Max Lowenhardt at home. Instead he spoke to Anna and asked her to pass the message that the Roths might be in danger in London and he needed to locate them as soon as possible. On hearing this, Anna decided to go to London immediately, hoping to find and warn the Roths, leaving only a brief note for her father.
On reading Anna's note, Max Lowenhardt also decided to travel to London, and departed for London one day after Anna. On arrival, he made his way to what he supposed was the residence of Dr Bruno Jacobs at Westwinds in Hampstead, hoping to find the Roths. Unknown to Max, Jacobs had moved to the country and the Roths were nowhere to be seen. Unfortunately Max had been followed )probably all the way from Berlin) by members of a gang of professional assassins sent out to hunt down the Roths. His visit to Westwinds had unwittingly led the gang to assume Dr Jacobs was residing at Westwinds. As Max approached the house, the gang members attacked him, probably hoping to take him prisoner for questioning. Fortunately, Algy was also watching the house and he intervened, allowing Max to flee in the ensuing scuffle.
Following this, Max managed to locate the address of Dr Jacobs, probably through enquiries at the professional society Dr Jacobs, who was a dentist, belonged to. Rather unwisely, considering the incident at Westwinds, Max traveled to Jacobs' residence at Saxton Old Hall in Surrey and again found himself accosted by another two members of the gang who were keeping watch over the place. This time Ginger intervened and drove the gang members off, allowing Max to enter Saxton Old Hall where he was reunited with Anna, who had been brought there by Ginger.
At the end of the story, it was stated that Max Lowenhardt and Anna emigrated to the United States where Anna married Moritz.