Major spoilers below, if you haven't already been spoiled by the rest of the Internet already.
Oh, where to start? How about the general episode itself? The Doctor finds herself trying to save an English town from Judoon searching for a target they've been contracted to find. After red herrings and plot twists, she discovers amazing and confusing things, survives multiple showdowns, and saunters off having rectified the situation at hand but with a head full of new questions yet to be answered. The base story itself was fun and well-executed, but it's the extras - the huge surprises and the callbacks - that made it special.
Of course, the big thing is the reveal of another Doctor, played by Jo Martin and who will be hereafter be referred to as the Ruth Doctor. (Yet another Doctor to confuse fan discussions. I will need to get popcorn for her numbering discussion once we find out how she fits in. I love this fandom.) She's shrouded in mystery, as neither she nor Thirteen remember being each other, so we have no idea if she's a past Doctor or a future Doctor. She's been arched to hide from Gallifreyans who have been trying to find her, and we leave the episode, well, with a head full of new questions yet to be answered.
And then there's Captain Jack. Who didn't squeal at that oh-so-recognizable voice coming over the loudspeaker on the spaceship? I turned to my husband and screamed, "Jack!" He'd been spoiled earlier in the day (thus reinforcing my vow to abandon Facebook every weekend until we can watch the new episode on Monday) and managed a rather convincing, "Nope," but there really wasn't any question in my mind. Jack was everything he should be: cheeky, sexy, and utterly dominating every scene he was in. He also gave Graham-who-he-thought-was-the-Doctor a long, passionate snog, which should vindicate all the Jack/10 shippers out there.
All in all, this episode was a big honking chunk of fan love. It gave you so much to think about - to try to figure out - while continuing to throw on new puzzles every other minute. And there's still so much to wonder about. But it also called back to so much, presenting previous plot devices with new twists while staying true to continuity.
The episode opened with a glimpse into Ruth's daily life as a tour guide in Gloucester - unable to attract customers because she delivers only dry facts - with her husband Lee who seems to be hiding something - mirroring the first scenes of "Human Nature". When the Doctor arrives and begins investigating her life, she uncovers more parallels: the mysterious filigree box as a substitute for the pocketwatch, though that's subverted by being a red herring; Ruth's inability to remember the details of her life; Lee's insistence that Ruth flee while he tried to hold off the aliens.
The true brilliance of this, though, is that instead of experiencing the invasion as the companion, through Martha's eyes, we experience it from John Smith's side because we don't know Ruth is the Doctor. Both John and Ruth were at the center of an alien invasion and saw people disintegrated before their eyes, but it's far more horrifying for us this time because we thought that Ruth was just a human, being deceived by her supposedly-alien husband (magnificent misdirection!). We fear for her because she's the innocent victim. John Smith was as well, but we knew he was the Doctor and we knew he'd get out of it; our concern for him was instead directed toward his sacrifice. (Note: I'm not saying this episode was better (or worse, for that matter) than HN/FoB; that story and this one had vastly different purposes and styles and thus warranted different approaches.)
Then, we have the Judoon, portrayed true to form as ultra-lawful police for hire, focused on their contracted task with no ability to peer beyond their blinders. It was heaven for this canon-lawyer to see them used correctly! Judoon are not actually evil or vindictive, and are not the Doctor's enemies - they should never have been at the Pandorica (unless they'd been paid to incarcerate the Doctor, and if they had, they should have been at the front, grabbing him to stuff him in there). These Judoon behaved as they were designed, to great effect.
Of course, the Doctor herself called back to "Smith and Jones" with her "Judoon platoon near the moon" and then "lagoon", establishing a clear connection back to her former self, but there were other parallels that were refreshing to see. I squealed (there were many squeals this episode) at the sight of the Judoon ships in space. The Judoon branding jumped a technology level from big Sharpies to X stamps. A resident of the town resisted with ineffective force, and was subjected to "Sentence: Execution". And lastly, I know this isn't a callback, but it felt like one: one of the TARDIS team (Graham, I think) called Jack's abduction of the three of them a "scoop", which just locked in the S&J feel. All of this was sweet candy to a faithful fan, while defining the essences of the Judoon and the chameleon arch for newer viewers who haven't seen two episodes from thirteen years ago.
Now, of course, we're wondering what's really going on and how this all fits into the apparent season arc that began with the Master's reveal that Gallifrey has been destroyed. I've got theories, which I won't talk about here, but this is what I want in a season arc: the slow reveal of information and clues all along the way as organic parts of the stories (not as asides like the Crack and Missy or discussions shoehorned into the plot), giving viewers enough to let them think, discuss, and speculate. Last season's arc, while enjoyable, was not as well-handled (I'm looking at you, "The Tsuranga Conundrum", with your Ryan and Yaz stopping in the middle of a six-minute deadline to have a heart-to-heart about Ryan's dad). The last time this was truly done well was Series 3 - perhaps it's fitting that this episode called directly back to it.
The next step for me is to re-watch this episode, to get a better feel for how it stands on its own. I readily admit that the heady excitement of seeing Jack and the chameleon arch (my absolute favorite DW trope) return could have overshadowed the actual value of the story. Hm. I might have to watch this two or three or ten times. :) Tentative score? 8/10 I'm scoring on the conservative side here.
Edit: Oh, I forgot. It's important to realize, after the fact: the Doctor's companion was brave to the end and was killed brutally. Lee's sacrifice doesn't really hit you until you know that Ruth is the Doctor - up until then, he seems to be just another human falling afoul of the Judoon. I love this. We've finally outgrown the immortality shield, the plot design idea that nothing truly bad will ultimately happen to the companion because the Doctor protects them, that was cast around the recent companions. We can have new Donnas, Jamies, Adrics, Marthas (and Martha's family), Peris, Sarah Janes, Tegans... Brilliant.