Tonight’s much-anticipated “Doctor Who,” perhaps my favorite episode so far this season, gives us our first glimpse of Arya Stark, well “Game of Thrones” actress Maisie Williams anyway, in the Whoniverse. It also provides an explanation for the 12 Doctor’s face.

Showrunner Stephen Moffat has hinted that he’d offer up an explanation for why we’ve seen the face of the latest Doctor, Peter Capaldi, on the show before – most notably as Roman patriarch Lucius Caecilius in 2008’s “The Fires of Pompeii.” And tonight he made good...