In Biggles Sorts It Out, Bill Carter is a South African police officer stationed at Windhoek, South West Africa, at that time a territory administered by South Africa. Carter calls on Biggles and Bertie at their hotel shortly after his arrival in Windhoek on the trail of Richard Browning. Carter had been alerted to Biggles' interest in Mick Connor by the airport manager Mr Grey. He tells Biggles that the South African police also had an interest in Connor and his partner Browning. He had information from a private source that Browning's plane had been seen over the Kalahari, and the police wanted to know what he was up to. Connor was a professional hunter but the police did not want him shooting game, especially ostriches from an aircraft. Apparently hunters had been shooting ostriches hoping to find diamonds in their gut as they swallowed diamoniferous gravel while eating. As there were tight regulations on the prospecting and sale of diamonds, Carter requested that Biggles share any information he gathered on Connor and Browning. He also offered Biggles any assistance he might need.
Carter proved to be of great help to Biggles. He checked and supplied information that Browning's aircraft had not been seen in any other airport within range of Windhoek, proving that Browning must have gone into the Kalahari. Also, in coming to South West Africa, Browning had been using a South African passport. Later, while discussing why Browning had acted so openly in his journey, Carter theorized that perhaps Browning wanted to be followed, to draw the police away from a confederate, and now that he was in the Kalahari, he would lie low--this observation was, it would turn out, very close to the facts. He also helped Biggles identify the fort he had overflown as "Fort Schwarz" and remarked that another site he had sighted in the desert was the fabled "Lost City".
At the end of the book, Carter was on hand to take charge of matters when Biggles and Bertie brought the injured Browning and the deceased Connor back to Windhoek from Fort Schwarz.