Scarlet Holmes is an agent for the S.O.E.
That well known acronym stood for the Special Operations Executive, started by MI6 by order of British Prime Minister Winston Churchill "to conduct espionage, sabotage and reconnaissance in occupied Europe against the Axis powers, and to aid local resistance movements" (Wikipedia).
The three recorded adventures that exist come from the stories recounted by Holmes to her great-granddaughter, Ada. That little 5-year-old seems to have trouble with bullies at school and Scarlet Holmes then tries to teach the girl that there are right times and right reasons to fight and she does it sort of by recounting her experiences during the Second World War while living in France.
How a 17-year-old girl from the swampland of Georgia, born in or about 1923, came to be in France working for both British Intelligence and the French Resistance is pretty simple. Well, how she got there is, at least, as we learn almost immediately in her recounting to Ada and to us.
Holmes' father had been a very successful moonshiner using the swamp as a great hiding place. He and his partner James then used the tremendous monies the Prohibition booze brought in to become extremely successful in the real estate business. He was gone a lot traveling the state and Holmes' mother used that time to be extra friendly with other menfolk and when Daddy learned of this, things got unpleasant. As Scarlet Holmes grew up, she showed she had the same sort of independent streak, not to mention a fondness for, well, fondness, and Daddy decided getting her out of the country was in her best interest so he booted her out with a trip to France.
Timing being important in most things, landing in Paris around the same time the Nazis came to call was what got Scarlet Holmes involved in the War because Scarlet being Scarlet, she tended to act on her emotions and that made enemies of the German invaders and rather than back down, she upped the ante by finding other ways to irk the heck out of the enemy. Hence the S.O.E.