Unless I missed something at the end of the previous season. it is not really clear why Frankie Drake is in England. For holiday? To meet up with old friends? To sort out the troubling aspects of her under-cover work during the war?
Whatever the reason for Frankie being in England, it makes for an enjoyable episode.
What makes it special is the early appearance of a relatively young Agatha Christie, played by an actress, Honeysuckle Weeks (whose name is almost as interesting as the character she plays!).
I found the Agatha Christie character to be one of the more interesting historical figures to have popped up on the show. Much more interesting, for instance, than I found Ernest Hemingway to be..
At the point that Agatha appears here, she has published a couple of mysteries already that have been popular., But she is not sure if she wants to continue writing that "foreign detective" she created.
As an amateur detective, Agatha proves valuable in several ways. And as a writer interested in people, Agatha proves able to "open Frankie up" a bit, so that we learn some about what is haunting Frankie Drake about her work during the Great War.
All in all, this was an well done episode, and would not be surprised to see Agatha Christie show up again on future episodes.