The moment I heard his disembodied voice, I knew that must be Captain Jack. I'm so happy that I was right, but damn it was just a big fat teaser. Great fun interactions between him, the three companions, and his slight confusion of who The Doctor was (but, of course, he didn't even bat an eye about The Doctor being a woman now).
...and yet...that wasn't even the biggest bombshell of the episode. I figured Ruth (or Lee before her) was a Time Lord in disguise and I was surprised The Doctor didn't figure it out sooner and started inquiring about a fob watch. Neat twist with it being a lighthouse this time around. But I definitely didn't see Ruth being The Doctor herself.
And now we have a big mystery, is she really The Doctor like she claims? Evidence for: Hartnell-esque TARDIS interior, fantastically weird attire; evidence against: violence against the Judoon and Gat. If so, then why doesn't The Thirteenth Doctor not remember (which would tie into The Master's claim of the lies)? The Ruth Doctor and Gat do seem to be genuinely in the past, even if that doesn't make any sense based on previous Doctor Who time mechanics (yeah, yeah, rules are changed any given moment).
Or it's some great elaborate lie by The Master, but that seems unlikely.
So has Chibnall really broken Doctor Who lore, for better or for worse? If The Ruth Doctor is genuinely before The First Doctor, does that mean there are others, like The Morbius Doctors (that'll make Allyn Gibson happy).
And why introduce Jack here and now without him getting to meet The Thirteenth Doctor? I have a feeling we're going to get even more blasts from the past before this season is over. Is it too much to hope for Romana at least?
I have to say, I'm very impressed how Chibnall and the BBC managed to keep both Captain Jack's triumphant return and a whole new unknown Doctor a complete secret. Although casting someone of far less pedigree than John Hurt certainly helps.