Popplewell was a major in the British Army under Colonel Gubbins during World War II. He was eventually promoted to general and wrote Reminiscences of World War II, published in 1946.
In an alternate timeline in which Germany won World War II, Popplewell (who preferred to be known as Pop) became the leader of the British resistance. He assassinated the Reichsinspektor General at the Festival of Britain. He later watched as the Doctor's TARDIS was taken away by members of the Britischer Freikorps, then told the Seventh Doctor and Ace what had happened. He recommended they visit Ma Barker's café.
The next day, Ma took Ace to the café (secretly a resistance HQ) where she again met with Pop and questioned the two of them about what had happened in Britain after the war and learning about the rumoured Black Coven. Arnold arrived at the café and, seeing Ace there, accused her of working with the Freikorps. Lieutenant Anthony Rupert Hemmings raided the café, but before he could arrest them, the Doctor arrived with General Otto Strasser and claimed that the four were double agents working against the resistance. (PROSE: Timewyrm: Exodus)