Judy

The Doctor and his travelling companion Bill Potts have a disagreement about the fate of the Ninth Legion of the Imperial Roman army whose mysterious disappearance has long baffled historians. The Doctor claims that they were all killed in battle but Bill claims to know all about the Romans than The Doctor and that they survived. They therefore travel in the TARDIS, along with Nardole, to the 2nd century in Scotland to prove who is wrong. Bill is convinced that the Legion are camped by the river in a nearby valley and so goes off on her own to bring one of the soldiers back to the TARDIS, while The Doctor and Nardole set off to look for their dead bodies.

Bill soon finds herself being chased through the woods by a young Pict girl called Kar. In her attempt to escape Bill falls down a hole and discovers some of the Legion's soldiers hiding underground. To her initial surprise Bill discovers that the TARDIS' translation circuits help her communicate with the soldiers. She learns from them that they are hiding from a creature they call a ‘Light Eating Locust’ that seems drawn to any light source, killing anyone in its path.

Meanwhile, The Doctor and Nardole discover the boneless corpses of the remaining Legion. They later come across a Pict tribe guarding a cairn and who are waiting for the return of Kar, their leader and the ‘Guardian of the Gate’.

The Argument

The Doctor impatiently enters the cairn, passing into an interdimensional portal full of creatures feeding off a light source. When he steps out seconds later, he finds that more than two days have actually passed. Nardole, thinking that The Doctor had gone forever, has integrated himself among the Picts.

Kar explains that once a generation, a warrior of their tribe goes through the cairn to defeat an 'Eater of Light', but with the invading Roman army, she allowed one to escape to fight them. The Doctor warns her that unless they can get the creature back into the portal and close it, more of its kind will escape and consume the sun and all the stars in the universe.

The Doctor

Meanwhile Bill leads the surviving legion away from the creature, that is attacking them, and they end up near the cairn, reuniting with The Doctor and Nardole. The Romans and Picts all agree to end their squabble and to work together to defeat the larger threat.

The Doctor works out a plan to lure the Eater of Light back through the portal during daylight. This though will require someone to stay within the portal until sunset to prevent the creature from escaping. But due to the time dilation of the portal, whoever does will age many years.

When the creature is trapped, The Doctor prepares to enter portal, explaining that as human life spans are too short he is the best candidate since his Time Lord physiology and regenerative abilities will protect him.

Bill Potts

Bill objects and knocks him down, while Kar and the remaining Ninth Legion instead sacrifice themselves as a group to stop the creatures from escaping, despite The Doctor's objections.

With the portal closed, the remaining Picts honour Kar's memory in monumental stones and by teaching the crows to say her name, which Nardole observes continues into the present day. Back in the TARDIS, Missy awaits their return, to the surprise of Bill and Nardole.