The Doctor, Bill, and Nardole uncover a monstrous answer to the mystery of Rome's Ninth Legion in a solid late-season episode of Doctor Who.

Season 10 of Doctor Who has been all over the place, both in terms of the actual locations (and times) of the Doctor's travels over the past 10 episodes, and in terms of the effectiveness of those stories. This late in the season there is an expectation that some sort of a larger arc is being built. While a framework exists, thanks to the vault and Missy, and the concerns she raised about the Doctor at the end of last week's 'Empress of Mars,' the show has mostly been keeping the Missy portions of the season's larger narrative in the background for the purpose of having something reserved for the final episodes.

Structurally speaking this makes sense. It's a familiar choice for anyone who's been watching Doctor Who during Steven Moffat's run on the series. But, given what we know about Moffat and Peter Capaldi's plans to exit Who this year, the expectations of season 10 are thereby altered in such a way that making an episodic trip to hunt a beast killing Romans this late in the game feels as if the show is perhaps diverting too much of its attention away from more interesting avenues. It's a little like the series is at odds with its episodic nature, and the result isn't necessarily fair to what is an otherwise serviceable monster-of-the-week story that also aims to add some Whovian insight into the disappearance of Rome's Ninth Legion.

That aspect makes for an entertaining jaunt, as the Doctor, Bill, and Nardole stumble upon answers to an historical question, by way of an interdimensional portal being guarded by a group of Picts who have seen their numbers decimated. Adding to the situation is what's left of the Ninth Legion: a group of young, surprisingly compassionate and understanding men who are, oddly, in much the same predicament as the Picts, thanks to a particularly savage interdimensional beast.

Pict Warriors in Doctor Who Season 10 Episode 10

'The Eaters of Light' makes for a swift-paced episode by separating Bill from the Doctor and Nardole, and using a bit of science fiction spackle to fill in things like potential communication barriers between Romans and young woman from England. No sooner has Bill been chased off by a warrior named Kar and fallen in with the Romans after being attacked by the mysterious tentacled beast than the Doctor and Nardole find themselves held at spear point by the rest of the surviving Picts – most of whom happen to be very young and seemingly inexperienced.

The division of Bill and the Doctor gives the hour time to breathe, despite keeping things relatively tense by way of the light eater that's skulking around just waiting to attack anyone foolish enough to venture outside. It's a siege story essentially, and writer Rona Munro – a writer who has worked on Classic Doctor Who episodes as well – works to tie the conflict brought about by the Roman Legion with the conflict inherent in a story involving an interdimensional creature that actually eats light. As the Doctor later notes, once he's had a good look at where the creature comes from, the beasts are just not just capable of making the days shorter, they'll eventually eat the sun and all the stars should they be allowed to run amok in his dimension. It's a welcome little detail that certainly raises the stakes of the hour and thankfully doesn't involve the promise of a more deliberate alien invasion, as that card has been overplayed in recent weeks.

Instead, Munro's script continually works to flip expectations on their ear. For starters, Bill's experience with what remains of the Roman Legion isn't hostile as you might expect. In fact, it's enlightening – for her. A discussion with Lucius, a particularly compassionate and attentive soldier leads to a discussion on sexuality, wherein it is Bill, limiting her preference to just one gender, who is seen as stuffy. Elsewhere, the Doctor takes on a more lecturing tone with the young Picts, one befitting his season-long (and increasingly ignored role) as a university professor. The Doctor comes across as curt and scolding when dealing with the Picts – especially the obstinate Kar, for whom the loss of her tribe and failure to guard the gate weighs heavily. The move effectively makes the Doctor the aggressor in the story, and while his attitude toward others is perhaps a little harsh at times, it is befitting of Capaldi's take on the character and it also helps to bridge the gap between the episode's end and the segment at the end, revealing he's freed Missy from the vault.

Pearl Mackie and Peter Capaldi in Doctor Who Season 10 Episode 10

Rather than a coda like last week's reveal that Missy helped bring Narole and the TARDIS back to Mars, 'The Eaters of Light' offers a little more insight into the Doctor's reasons for really breaking his oath (something he's been doing all season long, but still). Again, there is a surprising emotional undercurrent to the characters' decisions, one that sees the Doctor gambling on Missy having turned over a new leaf, and layering his hopes and expectations with the desire to think of her as a friend again one day. The emotional component is again surprising, especially given how the Doctor regarded the Picts early on. But Munro's script pays it off by first revealing the reflexivity with which the Doctor decides to spend an eternity guarding an interdimensional gate.

The Doctor can't be talked out of his decision, and is only prevented from entering the gate by physical force. His actions underline various aspects of his character and help shed some light (no pun intended) on his decision to free Missy (even though she's still confined to the TARDIS). It is perhaps one of the better examples of the seemingly contradictory elements of the Doctor's personality to be on display this season. And, in the end, the internal conflict in the Doctor helps what's to come feel a little weightier, as Capaldi's time in the role nears its end.

Next: The Next Doctor Who Has Already Been Cast

Doctor Who continues next week with 'World Enough and Time' @9pm on BBC America.

Photos: Simon Ridgway/BBC