Roots: Histories of Vlad Tepes of Wallachia. Naturally, Bram Stoker's Dracula is referred to (and dismissed), while dialogue drops in references to The Brides of Dracula and The Curse of Dracula. Peri compares her stitching to that of Frankenstein. The Doctor quotes from Edward FitzGerald's translation of The Rubaiyat of Omar Khayyam ("Awake! For Morning in the Bowl of Night Has flung the Stone that puts the Stars to Flight"). He calls his horse 'my beauty' - possibly a reference to Peter Davison's role in the modernised Black Beauty.
Dialogue Triumphs: Peri "What is it, Doctor? I'm all ears" "Well, nobody's perfect"
Continuity: Peri admits her sewing isn't up to much (although it improves over the weeks they are at the castle). At the beginning of the adventure she is wearing high heels. She is well-versed in modern vampire lore, but is unaware of the Vlad the Impaler aspect to the Dracula story.
Erimem writes to Peri in English, helped by the lessons she has given her.
The Doctor initially refuses the use of a sword, but when faced with no other options seems to acquit himself well with it. He seems to be familiar with the layout of Poienari [perhaps from his research?] He is also an expert marksman with a bow. At the end of the story he pays one of Radu's men to steal his master's journal and rips some pages from it - presumably he keeps it and is certain Radu won't simply start a new one.
Links: The Doctor tells Peri of having met vampires before "with fangs and bats and everything" (State of Decay) Erimem's English lessons continue apace (The Roof of the World)
Location: Wallachia and borderlands, June 17 to July 2, 1462
The Bottom Line: Historical stories in the modern Who era are a double-edged sword. Lyons has chosen the dramatic over the didactic by using one of history's most infamous butchers and asking whether the propaganda fitted the man, and if his brutal methods of war and population control were justified. Son of the Dragon becomes a character piece then, and with such speculation the documentary aspect of the historical leaves the room - perhaps that's the way it should be. There's a deft touch in making Erimem Dracula's sympathiser, although it comes at the cost of Peri being stupider and whinier than normal and Davison reverting to "Oh dear..." mode until late in the piece.