Doctor: Peter Capaldi (Twelfth Doctor)
Michelle Gomez – Missy
Rebecca Benson – Kar
Daniel Kerr – Ban
Brian Vernel – Lucius
Rohan Nedd – Simon
Ben Hunter – Thracius
Sam Adewunmi – Vitus
Billy Matthews – Cornelius
Aaron Phagura – Marcus
Jocelyn Brassington – Judy
Lewis McGowan – Brother
Directed by Charles Palmer
Written by Rona Munro
Produced by Nikki Wilson
Executive producer(s) Steven Moffat
Incidental music composer Murray Gold
Series Series 10
Length 42 minutes
Originally broadcast 17 June 2017
The Doctor and Bill, disagreeing about the fate of the Ninth Legion of the Imperial Roman army, travel in the TARDIS along with Nardole to the 2nd century in Scotland to prove the other wrong. Bill goes her own way to find the Legion, while the Doctor and Nardole look for their dead bodies.
Bill encounters some of the Legion's soldiers hiding underground, the TARDIS' translation circuits helping her communicate with them. The soldiers are hiding from a "Light Eating Locust" that seems drawn to any light source, killing those 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 waiting for Kar, their leader and the "Guardian of the Gate". The Doctor impatiently enters the cairn, passing into an interdimensional portal full of creatures feeding off a light source. He comes out seconds later, but finds that more than two days have actually passed. 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.
Bill leads the surviving legion away from the creature and end up near the cairn, reuniting with the Doctor and Nardole. The Romans and Picts all agree their squabble is childish given the larger threat. The Doctor works out a plan to lure the Eater back to the portal during daylight. Once the creature is trapped, the Doctor tells them someone needs to stay within the portal until the sun extinguishes to prevent the creature from escaping, but as human life spans are too short, he prepares to enter the portal himself, since his Time Lord physiology and regenerative abilities will protect him. Bill objects and knocks him down, while Kar and the remaining Ninth Legion instead sacrifice themselves as a group to stop the creatures, 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.