WMG / Doctor Who S36 E4 "Knock Knock"

• In the first episode there was a picture of Susan predominantly displayed on the Doctor’s desk. This week we had Bill repeatedly call the Doctor ‘grandfather’ - so I’m calling it now, either Susan is in the vault, Bill is really a regenerated Susan or she plays a role somewhere in the final story.
  • Susan wouldn't cheerfully play 'Pop Goes the Weasel' upon hearing of the deaths of several young humans. It's obviously the Master / Missy.
  • I'm going to guess it's the previous incarnation of The Master, and that The Doctor will, in a Moffettesque Stable Time Loop, be responsible for his regenerating into Missy.
    • All Jossed, from later episodes confirming who is in the vault.
• Episode writer Mike Bartlett is a webcomics fan. The initial setup, with a group of young adults in need of a place to live who find a creepy landlord with a spooky house (complete with forbidden upstairs area) and a price that's too good to both ignore and be true, is very similar to the Kesandru House arc in Sluggy Freelance. This is not a knock on either the writer or the episode, as Who has always worn its influences on its sleeve. Both houses are destroyed in the end, but the circumstances are very different. Eliza in the tower is very different than the ghost of Brad Trivol in the locked-and-boarded attic room, for starters.

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