The Doctor, Sam and Fitz land in the Belgian Ardennes in late December 1944
, after materializing blind for repairs to the TARDIS. After the TARDIS falls into a deep river from a collapsed snow-bank, Sam gets separated from Fitz and the Doctor (as per usual) and the three must find a way to get back to each other, retrieve the TARDIS and escape before the war proves fatal for one of them.
Tropes present in Autumn Mist include:
- Back from the Dead: Sam is actually killed when the prison camp she's been sent to gets bombed. Luckily for her, The Fair Folk like her.
- Came Back Strong: The sidhe bring back Sam and have to patch her up with some kind of prosthetic skin, which stands out noticeably against her real skin. It makes her much more durable in general, but Sam still thinks of it as Came Back Wrong.
- The Fair Folk: Sort of.
- Kids Are Cruel: Fitz's childhood memories of being bullied for being half-German in the years directly following the war get brought up again.
- Sorkin Relationship Moment: Fitz keeps thinking Sam is going to spring one on him for sleeping with Dark Sam, but near the end of the book, she goes to the Doctor instead. And tells him she's leaving.
- Stupid Jetpack Hitler: Though in this case, the Allies and the Germans are racing to get the alien weapons, and nobody has them yet.