I enjoyed this standalone episode. At first, I thought it was going to be a straight up horror episode, but it blended in sci-fi as it went along.
The adventure results from Bill and The Doctor betting on what happened to the Roman Legion. They find out that a young warrior named Kar unleashed a monster from a portal to destroy the Ninth Legion, while they weaken the monster. Her plan would have been beneficial if there weren’t hundreds of other monsters on the other side of the portal.
This episode highlights how young the tribe of warriors and Roman Soldiers are. It also offers a critique of how empires look to those on the outside. After The Doctor tries to sacrifice himself as the guardian of the portal, the factions that are fighting end up teaming up to guard the portal and keep the monsters out of their world.
At the end of the episode, it is revealed that The Doctor is letting Missy hang out in the Tardis instead of the vault. She is doing some maintenance for him. Bill and Nardole are not happy about it. The Doctor mirrors the audience in trying to decide if Missy is putting on a facade or is sincerely changing.
The last two weeks have focused on resolving military conflicts without all out war. The show has done a great job of showing a different perspective on conflicts and how small they actually are when looking at a cosmic level.