The Doctor (Peter Capaldi) and Bill Potts (Pearl Mackie) are having a disagreement about the mystery of the Ninth Legion of the Imperial Roman Army. Don’t we all? So, they take the TARDIS to Scotland in the 2nd Century to settle their differences. Bill goes off in her own direction, and the Doctor takes Nardole (Matt Lucas) with him to look for bodies. Bill finds some of the Legion’s soldiers hiding underground. They fear the Light Eating Locust that kills everything in its path to get to light. The Doctor and Nardole find the corpses of other legionnaires, their bones missing. They also run into a tribe of Picts guarding a cairn and waiting for Kar (Rebecca Benson), their leader, whom they call the Guardian of the Gate.

The Doctor passes through the cairn into an interdimensional portal full of creatures feeding off a light-source. He comes out a few seconds later to find that two days have passed. Kar tells him that once every generation a warrior goes through the cairn to defeat the Eater of Light. But she allowed one to escape, hoping it would defeat the Roman invaders. The Doctor warns her that they must seal the creature back into the portal or its kind will consume the sun and the stars.

Bill leads the surviving legionnaires away from the creature to the Doctor and Nardole. The Romans and the Picts agree to unite against the greater threat. The Doctor has a plan to lure the Eater back through the portal. Someone must stay with the creature to keep it there, but humans have too short a lifespan and he needs to enter the portal himself. Bill objects and she knocks him down, while the remainder of the Ninth Legion enter the portal, sacrificing themselves. The Picts honor Kar by teaching the crows to say her name. Now, that is all they say, because they are in a snit and won’t speak to humans anymore. Back in the TARDIS, Missy is waiting, apparently worried about the Doctor.

The story was written by Rona Munro, who wrote the final episode of the classic series, called Survival, for the Seventh Doctor, Sylvester McCoy. Thus, she was the first writer to work on both the classic and revised series. It was considered a flawed but solid episode. When Kar says, “The Romans’ work is robbery, slaughter, and plunder. They make a desert and call it peace,” she is referring to a speech written by Tacitus. The Doctor tells Kar that she got the deadliest creature on the planet very cross, but does he mean the Eater of Light or himself? At one point, they find the image of the TARDIS carved in stone.