Okay, this was a hit-and-miss episode co-written by (wait for it) Stephen Moffat, which explains why it's so hit-and-miss.
The premise: After narrowly rescuing Clara from certain death—she's floating in space with a brain-eating spider-thing crawling through her environment suit—our two heroes land in a Medieval Viking village and are quickly captured by the local warriors. When taken back to the village, the Doctor tries rather pathetically to impersonate Odin to a disbelieving crowd when another fake Odin appears, teleports all the warriors away along with Clara and Ashildr (Maisie Williams), a village girl who is the character referred to in the episode's title, and the warriors are summarily murdered, leaving Clara and the girl to go back with a message that in twenty-four hours the aliens, called the Mire, will attack and kill the entire village. The Doctor must then train a bunch of farmers, fishermen, and storytellers in the art of combat if they are all to die with honor, because really, there's no hope of winning (or surviving) by fighting.
The rest of the episode is the Doctor, minus TARDIS or sonic sunglasses, saving the day, and the Time Lord realizing why he chose the face he did, a face that's appeared twice in the Whoniverse. For some reason Moffat felt obligated to offer up such an explanation, never mind that it's been done before in the Classic series, first with actress Lalla Ward stepping in to play the Time Lady Romana after playing another character, and then with Colin Baker playing the Doctor after previously portraying another Time Lord during the Tom Baker era. Oh well. But it does add another layer of personality to the current incarnation, so yeah, decent enough idea.
We also get hints that Ashildr will return again in Series 9 and that she will be an enemy of the Doctor next time.
What I Liked:
Clara is restored to something resembling a strong, smart female lead in the show, which is good. So far in this season, the character has been treated like a simpering comic relief moron, which cheapens both her and Jenna Coleman, the actress who portrays her.
Classic series references harkening back to the 7th Doctor era, in which the Doctor describes ripple effects of meddling with history, and that these ripples lead to unforeseen consequences.
What I Didn't Like:
I get that both Russell T. Davies and Stephen Moffat are atheists and that they're not at all keen on religion. But why do they and writers on the payroll in the revived series keep having the Doctor make some disparaging remark on people's faith in something more? I'd like to see an episode in which the Doctor and his companion(s) are challenged to at least question if there's some guiding presence in the universe that operates in ways so subtle we often miss it.
Overall, because Moffat co-wrote it and brought with it all the usual storytelling problems, I give this episode a B-.