Have we ever seen the two Victorian adventurers before? I really don't remember them but I missed a lot of 6th-7th doctor episodes. If not, the maid was called Jenny - could she be the Doctor's daughter after a regeneration?
I think we've answered my question a week or two ago about Rory's swordsmanship - now I really want to write that Highlander crossover, since Jenny and her friend, The Centurion, and Duncan MacLeod must have all been active in Victorian London around the same time.
Later - I'm an idiot, the Centurion was only active in London in the timeline that was destroyed, which can't be the same one the two ladies come from.
later still - Except that the fighters also came from that timeline. Okay, wibbly wobbly timey wimey stuff I assume, they can coexist.
Stonkingly good episode anyway - now I want to see what happens to Hitler next time around...