I did notice that it was written by Jamie Mathieson, who also wrote my favorite episodes from last season, "Mummy on the Orient Express" and "Flatline". More on that later.
The Doctor and Clara land in a Viking village that's about to be attacked by an alien menace. Despite what seems to be insurmountable odds, the Doctor pieces together all of the strengths of the village, adds in a bit of future technology, and defeats the aliens without bloodshed. Great story, good adventure, lots of the Doctor being arrogant and disdainful, as well as clever. The pacing was great, building up the tension and the excitement well. The guest characters except for Maisie Williams' were poorly-drawn, but I've come to expect that in DW now, so it doesn't bother me as much as it used ot. ("Mummy", I think, broke me of that.)
The bits in between? Not so much. The moral discussions were so heavy-handed, and Twelve moped so much, it was terrible. It was the same way in both "Mummy" and "Flatline": the adventure was great, but the interwoven story (Clara taking the Doctor to task for lying, Clara trying to be the Doctor) felt stapled on, as if the concepts were given to the writer and told to force it in. I have never seen Mathieson's work outside of DW and I wonder, is he simply good at adventure and poor at character/relationship, or are all the things I don't like about the three episodes Moffat's stuff?
I loved the callback to "The Fires of Pompeii" (one of my favorite episodes of all time), until I realized that they completely missed the point of the final scene. Donna's point was that yes, the Doctor had to choose to kill everyone, but if he pulled himself out of the mire of his regret and opened his eyes, he could do one bit of good by saving someone. Somehow, that "just save someone" line got translated into "resurrect the one person that died." Resurrecting her actually ruined the story for me (though more on that later). Also, saving that one person at all costs reminded me of the Doctor taking it upon himself to make decisions for everyone else - the first thought that came to mind during this scene was Ursula, and then Donna. What was it that Mr. Copper said? "But if you could choose, Doctor, if you decide who lives and who dies, that would make you a monster." But I suppose, that was the Tenth Doctor's storyline - the whole point was that he was that monster, and he tries to suppress that until much later in his life. Here, it's used to make the Twelfth Doctor a hero, and to me, that's a terrible hero.
I am glad though, that there's actually going to be consequences for the Doctor's actions, and that saved the end of the episode for me (though again, it felt completely tacked on to the adventure). It's pretty terrible that he gave her the alien bit without thinking, and then gave her the ability to doom someone else to the same state if she wishes. The dialogue did imply, though, that she can be killed if it's done thoroughly enough, and hopefully that will be accomplished in the next episode.
Side note #1: What's so great about the "greatest warrior race in the universe" if all they really are is dudes in armor with a chip that heals them as fast as they take damage? THAT is the thing that the Doctor should have threatened to broadcast to the universe.
Side note #2: I've only seen Maisie Williams in two things, GoT and this, and I really don't like her. I have only seen the first three seasons of GoT and I thought I didn't like her because of her character (so far, as far as I've seen, Arya hasn't done anything other than run and threaten enemies far stronger than her and then need to have someone save her; oh wait, that's what this character did, too). She's great when her character needs to display an extreme emotion - anger, happiness, fear, etc. - but when she's just a normal person, she delivers her lines like she's reading off a card.