This episode of Doctor Who finds the TARDIS crew going back to the time of the ancient Romans and Scottish Picts. A period in history that the show has visited a few times over the years starting with the First Doctor who was accidentally responsible for the burning of Rome. This story begins with a scene from modern times with two children exploring the Scottish countryside and discovering the remains of an old stone cairn. Ghostly music can be heard whilst standing in the ruins.

The story explores the legend of the Ninth Legion from the Imperial Roman Army that disappeared without a trace. To this day no one is really sure what happened to them, and the Doctor wants to find out once and for all. When they arrive in ancient Scotland, the Doctor, Bill, and Nardole are discussing the Ninth Legion and more importantly, if they were all killed in battle, then where are all the bodies? Bill gets the idea to search in one direction, while the Doctor wants to go in another, so they separate and go off on their own with Nardole following the Time Lord. I guess they feel Bill can handle herself wandering around what is technically a war zone. She even promises to bring back a live soldier.

Bill finds a lone Pict warrior mourning the dead lost in battle. She turns to find Bill watching her and gives chase. For the second time in two episodes, Bill falls down a hole. While the Doctor and Nardole are wandering around, a crow cries out and Nardole thinks it has spoken to them. The Doctor assures him that crows do talk, even in the future. It is the humans who have stopped listening. They find an old stone cairn and decide to take a look. These places are supposed to be doorways between worlds.

Meanwhile, Bill has fallen down into a small cave where she finds a live Roman soldier from the Ninth Legion named Simon. They get into an argument about how they can understand each other. Bill postulates that it has something to do with the TARDIS being able to translate. Simon the soldier has no idea what she’s talking about but goes along with her explanation anyway.

Near the stone cairn, the Doctor and Nardole find the body of a Roman soldier. The corpse has been shriveled and disintegrated. According to the Doctor, this is obviously the work of some nefarious alien menace since this body hasn’t been dead for long. Over the next hill, they find out that the entire Ninth Legion has been killed in a similar fashion. What could possibly do this? When Bill and Simon emerge from their hole in the ground, they are pursued by a voracious beast. The design of the creature was well done with CGI. It’s covered in tentacles colored with glowing blue stripes which give it a peculiar effect as it plods along the hillside. As Simon takes Bill to where the rest of the still alive soldiers have hidden, the monster catches up to them and drags Simon to his death. Before his demise, he tells Bill where to find the rest of the survivors. Bill finds the cave and is pulled into safety, but the monster has found the cave as well. The soldiers quickly block the entrance so it can’t get in, then they settle down to finding out who this strange woman is.

The Doctor and Nardole are captured by the Picts. They are all teenage warriors with no surviving adults. The young woman who chased Bill away is Kar, she is the keeper of the gate, and the leader of this tribe. These warrior children maintain that is was Kar who defeated the Romans all by herself, but it is obvious they are not telling the complete truth. The Doctor and Nardole cause a distraction and are able to escape. They make it to the cairn where the Doctor ventures inside. There is a portal to somewhere unknown, and after a few seconds of examining it, the Doctor goes back outside.

Nardole and the Picts are grouped outside. Nardole is telling the Picts the story of the Mary Celeste and how the disappearance of the crew was the fault of a race of aliens known as Enzomadons. (Nitpick – In the First Doctor story, The Chase, it is clearly shown the Mary Celeste incident was the fault of the Daleks.) When the Doctor emerges from the cairn, Nardole tells him they have been waiting for him for two days, even though the Doctor was only in there for a minute. Nardole also tells him they cannot find Bill.

The Doctor asks Kar if she knew about the portal. Every generation of their tribe has a gatekeeper who fights the monsters and keeps them on the other side of the portal. The Eaters of Light, Kar calls them. But this time, an Eater has broken out and is on the loose. As the Doctor offers to help, someone screams and they run off.

Bill is in relative safety as she has made a friend with a Roman soldier named Lucius. Not only are they the survivors of the terrible battle which almost wiped them out, they are also still teenagers with Lucius being the oldest at eighteen. He wants more than friendship and Bill tells him she doesn’t swing that way. Lucius isn’t too hurt and tells them that Vitus, a fellow soldier is gay. Now, this could be construed as filling a historical story with modern anachronisms about sexuality, but in fact, the ancient Romans were pretty cool with gays and lesbians. Unfortunately, their ideas died along with their empire. So along with sanitation, medicine, education, wine, public order, irrigation, roads, the aqueduct, and public heath, the Romans also gave the world LGBT acceptance. Lucius even goes so far as to tell Bill she’s “really sweet for being so restricted.”

The Doctor and the Picts have found the withered dry corpse of one of their own. The Doctor determines that the longer the beast roams free, the stronger it gets. Kar admits to releasing the beast so it would kill the Romans, not realizing it would turn on her people and now it’s going to kill everybody. Meanwhile, Bill is telling the Romans about the Doctor and how he can help. They’re too scared to leave the cave and search for help, but Bill gives them a little pep talk and restores some of their confidence. The Doctor, Nardole, and the Picts are discussing the portal. If they open it a little every year, it prevents the portal from tearing apart and causing major damage to the surrounding area, but that is how the Eater escaped. They have to lure the beast back through the portal before sunrise when it will grow too powerful to defeat.

The Romans take Bill through the caves to a spot where they know the Picts to be above in their own hiding place. Her plan is to bring both sides together. She is certain the Doctor is with them but urges caution. One soldier doesn’t listen and rushes forward to find the beast waiting for him around a corner. He’s dragged off to his death while the rest escape right into the Pict’s hiding place. Now we have a nice big reunion and another discussion about how everyone can understand each other. The Doctor brings both groups together in order to fight the monster.

The Doctor formulates the following plan: Thanks to the time dilation of the portal, one person can spend a few minutes fighting the monster in the portal while decades pass outside. (We assume that the battle ends with the warrior’s death.) Since the Doctor can keep on regenerating instead of dying, he can hold the monsters off permanently. Bill argues with him saying that this is not his battle to fight. The Picts and the Romans all agree they should enter the portal and keep fighting forever.

The plan works, the monster is forced back through the portal where it can be held and fought. The Romans and the Picts tell the Doctor “Thanks, but no thanks, we will fight our own fight.” Both Romans and Picts enter the portal as the cairn begins to collapse. They will be fighting the Eaters of Light for eternity. The Doctor, Bill, and Nardole run to safety. The Doctor laments the loss of more people for something he could have fought himself, but he accepts it and moves on. Then we find out why the modern-day children from the beginning can hear bagpipes playing.

Back in the TARDIS, Missy is hanging out like she lives there. This time it is the Doctor’s doing. He is giving her a little freedom in exchange for her skills in TARDIS maintenance. She can’t leave the TARDIS or work the controls, but she can make repairs. The question arises if the Doctor can begin to trust his old adversary, can they ever be friends again? This is the crux of what will transpire during the season finale two-parter, and hoo-boy does it look good!

For me, this story was good, but nothing spectacular. Although the cast gave solid performances all around and the episode was certainly enjoyable, there wasn’t much emotional impact. Which is fine because I think Moffat and crew are about to give us the television blockbuster of the year in the show’s season finale.