When Henrietta doodles, she always sketches a stylized tree that she calls Yggdrasil. In Norse mythology, Yggdrasil is a giant ash tree that represents Viking cosmology, with the branches standing for different parallel worlds. "Heaven" is at the top and Hel is at the bottom. The world we experience is on one of the middle branches.
The Yggdrasil doodle looks exactly like it does in the book
Edward Fox's (Gudgeon) younger brother James Fox appeared in the previous episode Death on the Nile.
This is the first depiction of sexual activity in the entire run of the television series.
Sarah Miles, who plays Lucy Angkatell, had an affair in the 1960s with the actor James Fox. His brother Edward Fox plays Gudgeon in this episode.