In Biggles Nets a Fish, Doctor Mills was the deputy director of the atomic research station at Heatherstone Moor. When Biggles arrived to investigate how a micro-film of top secret documents came to be made, and was then smuggled out of the station and air dropped over Germany, Mills received him and conducted him on a tour of the establishment and assisted in the investigations.
Later, Biggles uncovered the role of the station's neighbour Felceman as the courier for the micro-films. The micro-films were being dropped into the stream which passed near the station and then retrieved by Felceman who lived downstream. At Biggles' urging, Felceman made use of a pre-arranged emergency procedure to unmask the spy who supplied the micro-film. After making a pre-arranged set of light signals, Felceman took off in his Gipsy Moth and on the airstrip inside the research station. The plan was that the enemy spy would meet the aircraft and together they would escape. The emergency procedure was executed with Biggles hiding inside the Gipsy Moth. Upon landing, it turned out that it was Doctor Mills who was waiting to board the aircraft to escape! Having thus been unmasked, Mills drew a knife and attacked Biggles but Biggles tripped him and then Felceman pinned him to the ground. Finally, after a brief struggle, Colonel Barclay, the station security officer arrived to arrest Mills.
Mills' motivation or ultimate fate was not revealed in the story.