O’Dell’s The Hound of Justice (Harper Voyager, July), her second Janet Watson chronicle, features very different versions of Conan Doyle’s doctor and detective.

Where did the idea for the series come from?

Fanfic was my starting point—an experiment that turned into a series. My Watson and Holmes didn’t have to be two white men. What if they were both women? What if the war was a new American civil war? And since a point-of-view character should be the one with the most at stake, then clearly a black woman, especially a black queer woman, would have the most to gain, the most to lose, in a U.S. civil war.

Had you always planned for Sara Holmes to be secondary?

Yes. I thought Doyle short-changed Watson, who existed purely to admire Holmes and chronicle his genius. So I gave my Watson a detailed backstory: a family that loved and quarreled, a lover who had abandoned her, a past layered with emotions. And when Janet Watson returns from the war, she isn’t a vet in name only, with an unspecified wound the story can conveniently ignore. She has PTSD and a badly fitted mechanical arm. Her life and her career have been destroyed, and she’s determined to rebuild both.

How much can Holmes and Watson be changed while still being recognizable?

Just as we can take King Arthur or Hamlet or Romeo and Juliet, turn them inside out, and still retain the essence of what makes them who they are, we can do the same with Watson and Holmes. So we end up with TV’s Elementary—Watson as a Chinese-American woman; or Aliette de Bodard’s The Tea Master and the Detective—far-future science fiction with Watson as a mindship. What ties them all together are the two archetypal characters—the genius detective and his friend the doctor—the mysteries they solve, and the strong bond of friendship between them.

How did you plot the circumstances that led to a second civil war?

I started off with the idea the U.S. was due for a second civil war. Even though this was back in 2014, the clues were already there. Fascism and white supremacists had already made the transition from fringe groups to the mainstream. These people were effing angry that the president was a black man and that gays and minorities were finally achieving equal rights. With those details in mind, I mapped out what happened in between. After the 2016 election, I tweaked the backstory to fit in Trump, but it was scary how little I really needed to change.