The episode revolves around the disappearance of the Roman Ninth Legion in the second century. Personally, I enjoy when Doctor Who takes an actual historical event and builds a story around it. The Romans (ever the invaders) are fighting the Celts in Scotland; however, this particular tribe already has a fight of their own. They are the protectors of a gateway where every so often, a monster will come through that they must defeat. Time works differently on the other side of the gate, so while it is only moments between the creatures leaving their world, years pass for us (like an alien Brigadoon).
This generation of Celts decide that instead of fighting their monster, they will let it through to fight the Romans in the hopes that both will annihilate each other. As so often happens when people use one monster to fight another, their problems only get worse. Both sides realize that they are facing a threat greater than each other, and they unite to stop it.
Part of how they are able to work together is that the TARDIS translation circuit allows them to communicate with each other. I enjoyed that Bill is able to figure out the basic idea of this without any sort of explanation from the Doctor; however, she has been on several adventures at this point, and it is strange that it has not come up until now. The TARDIS is telepathic, and the translation circuit works like a Babel Fish to translate for the operators. There is one major problem with this, though: Neither the Celts nor the Romans ever enter the TARDIS to get their telepathic upgrades, which means that there is no reason why they would be able to understand each other.
This week’s episode was fun, but the preview for next one appears to be far more exciting, as Missy claims to be on a path of redemption. We will finally see the return of the Mondasian Cybermen and John Simm as the Master.