Once again, sacrifice plays a big part in this week’s Doctor Who, as second-century Scots and Roman soldiers have to deal with a deadly alien. There’s also another example of someone making a big mistake that could have been bigger.

SPOILERS BELOW

It all begins with two Scottish kids at the “Devil’s Cairn.” A girl wants to hear music that comes from the hills. Her brother claims there are very hungry ghosts. They don’t notice a rock nearby with a raven (not from Game of Thrones) saying “Doctor”… and a TARDIS drawn on the rock.

It turns out this trip to the second century was based on a bet over who knows more about the Ninth Legion and what really happened to them. Bill thinks she’s an expert after reading books, while the Doctor says he lived there as a farmer, juggler and governor.

Bill literally stumbles into one of the soldiers, Simon (Rohan Nedd). She’s surprised to learn they can understand each other, thanks to the TARDIS’ automatic translating powers.  Simon is just stunned. He tells her a monster attacked his group, and the Doctor learns this, too. It has tentacles and literally eats people and light.  It somehow got to Earth through one of the Cairns, guided by a young girl named Kar (Rebecca Benson). She made the mistake of unleashing the monster on the Romans and made it upset. She thought she was saving her people from Romans who pillage, steal and kill. The Doctor also learns the cairn is near a temporal rift where time is bent. Using cairns as “gates” between time portals sounds a bit too much like Outlander, except nasty monsters are involved rather than romance.

What’s interesting about this battle is that no adults are involved, just young soldiers and even younger barbarians. They both act brave, but the Doctor can tell they are even more scared, especially after another soldier is killed. So, he suggests they can either kill each other through their hate or “grow the Hell up.”

Through some special magnifying glasses, they are able to force the monster back through the gate. Now they need someone to guard it, and the Doctor thinks he should do it because he can live for eons. Never mind that he’s supposed to guard a vault with Missy (Michelle Gomez), supposedly, inside.

Kar and the Romans think otherwise. They’re willing to sacrifice themselves to hold off the monster for all time. Kar’s brother vows the crows will say her name, and the music from her people will be heard. The Doctor still thinks he should guard the gate, but Bill and Nardole pull him away. He’s got another job to do.

Two other scenes were pretty good. First, Nardole (Matt Lucas)explains to Kar’s people how the crew from the Mary Celeste disappeared because of an unfortunate meeting with aliens. Then, Bill tries to let Lucius (Brian Vernel) down easy when she thinks he’s coming on to her. His response is a little surprising to her. Let’s say he’s not upset she’s gay because that’s not unusual to him.

Afterwards, the crew is very surprised Missy is still in the TARDIS, actually giving it some much needed maintenance. She was also interested in what the Doctor did. He just said that grasping the universe is one thing, but really listening to its music is another. That young girl at the start of the episode could hear the music.

So does Missy, and she sheds another tear. She even suggests that maybe they can be friends again. The Doctor is intrigued by that possibility, but it’s safe to say something will upset that.

Or maybe she doesn’t mean it.

This episode was pretty good, especially the Doctor’s speech to the survivors and Bill talking about sex with the Roman soldiers. It could have been a throw-away episode before the two-part finale, but it was a solid episode about how time travel can unveil some hidden history.

There wasn’t too much to the monster, though. Aside from its light-eating power, it looked like a cute offspring from one of the dragons from Game of Thrones.

Next week will be the beginning of the end for the 12th Doctor, as he sees some very familiar faces.