No spoilers ahead.
There are, like last episode, stories that slot into Moffat’s overall story and then there are … just stories.
I had to wrack my brain. That’s David Warner. He once played Jack the Ripper, in the movie Time After Time.
Kudos to Gatiss for putting Vienna in this episode, (Believe it or not, I first heard it just last year, via a cover on Spotify. I love that song. OK, so my backlog is that long…)