At the Devil's Cairn in Scotland, a young girl named Judy runs to a field of stones and tells her brother that she wants to listen to the music. He warns that she'll get him into trouble, and Judy insists that he'll hear it in a moment. Her brother pulls her up and warns her that she listens to ghosts in Hell, they'll eat her. As they go, Judy hears the music and goes back to the stones. Inscribed on one of them is a picture of the TARDIS.
Sometime later, the TARDIS materializes in the same field in 2nd century AD. Nardole wonders why traveling there is more important than the Doctor keeping his oath, and the Doctor says that Bill thinks that she knows more about the Romans than he does. They argue about whether the Ninth Legion were annihilated in battle or not, and says that they would have followed the nearby river. The Doctor tells her to check the river while he checks their last battlefield, and goes with Nardole.
Bill enters the forest, unaware that something is watching her. A local girl, Kar, is sitting at a fire honoring her dead mother. She looks up, sees Bill, draws her swords, and screams. Bill runs as Kar charges at her, and she falls through a hole into a cavern below. A Roman soldier is there, holding a sword on her.
The Doctor and Nardole look for signs of the soldiers, and Nardole sees a crow perched on a stone. It says Kar's name, but the Doctor is too busy finding a stone cairn indicating Pictish civilization. The Doctor dismisses Nardole's concerns and they continue, as more crows fly up.
The soldier, Simon, points out that Bill is speaking Latin even though she hears him as English. She realizes the TARDIS is translating, and Xxx says that he's with the Ninth Legion. He asks if she knows what happened to them.
The Doctor and Nardole find a centurion's corpse. His bones have apparently disintegrated due to the complete absence of sunlight, and it would take decades. The Doctor figures that it's alien and wonders where the rest of the Ninth Legion is. They find the corpses of the rest of the centurions, and the Doctor realizes that it occurred only a short time ago. He tells Nardole that they need to find Bill. A group of Pictish children arrive and order them to surrender.
Night falls, and a creature moves through the forest. Meanwhile, Bill climbs out of the cave and then helps Simon up. They hear the slime-covered creature growling and Simon says that it's what slaughtered the Ninth. Bill doesn't see anything, and Simon says that he has to get back to the handful of survivors. He leads Bill to the survivors, and explains that they were the deserters who couldn't face the monster. The monster approaches them and Simon tells Bill where the survivors are as he prepares to fight. The creature snares Simon with glowing tendrils and drags him to it, and Bill runs. She follows Simon's directions and finds the cave where the survivors are hiding, and crawls inside. A soldier, Lucius, is waiting with his sword drawn. The monster grabs her shoulder and Lucius and the others quickly pull Bill the rest of the way in. She joins them throwing rocks to drive the creature back, and they cover the entrance.
The centurions take Bill to their fire and confirm that she isn't one of the Pict. They ask if she saw Simon, and Bill tells them that he's dead, and the centurions realizes that now that the creature knows where they are, it will kill them all. Lucius insists that no one is going to die, and Bill collapses, a black piece of slime on her neck.
At the Pict camp, Ban tells the prisoners that Kar, the Keeper of the Gate and his sister, will soon be there. The Doctor warns about the monster, and demands to know where Bill is and what destroyed the Roman Army. Kar comes in and says that she destroyed the centurions, and explains that she was gone because she had to find the Gatekeeper's things. The Doctor asks where the adults are, and Kar says that the Romans are the robbers of the world. She talks about how the Romans rob everything in the name of Empire. The Doctor points out that they're all dead and doesn't think Kar killed them. He wonders what the Gate is and what's on the other side, just as the popcorn he's tossed in the fire goes off. In the confusion, the Doctor grabs Nardole and escapes.
The Doctor and Nardole go to the stones looking for the greatest danger. He figures that the cairns are gateways to other worlds and enters one. Kar and the others arrive and grab Nardole, and the ground. The girl tells Nardole that his friend won't be coming back.
Inside the cairn, the sunlight from the dawn shines on a section of wall. The wall opens, revealing an energy portal leading to an ocean with fish swimming in it. One of the creatures swims toward the portal, and the wall closes shut. The Doctor goes outside and finds Nardole, now wearing Pictish blue paint, telling the children a story about aliens. Nardole sees the Doctor and explains that he was in there for two days, even though only seconds passed for the Doctor. The Doctor realizes that the portal is an interdimensional temporal rift, and asks if Nardole has found Bill. He admits that he didn't find her, and they and the children go off to find Bill.
In the cave, Lucius wakes Bill up and gives her some food. She says that she needs to find the Doctor, and Lucius assures her that they'll look after her.
Kar returns and the Doctor tells her about the rift. She explains that the Eater of Light is on the other side, and every generation they've held it back. A new warrior goes to the Gate and fights the Eater so it couldn't break through. But it did break throw and destroyed the Ninth Legion. Kar insists that it's weak and will die soon, and the Doctor tells her that there are millions more on the other side. She vows to hold them back and says that she doesn't want the Doctor's help, but he tells her that he's all she has. Someone screams and Kar runs off to investigate.
Lucius says that Marcus was touched by the slime but sunlight cured him, and Bill will be all right. Bill realizes that Lucius is romantically interested in her and tells him that she doesn't like men, and Lucius realizes that she's like one of his soldiers, Vitus. He says that he likes men and women, and thinks that it's sweet that Bill is so restricted. The sunlight shining in is cut off, and a centurion Cornelius warns that the Eater is trying to find a way in.
The Doctor and the others find a desiccated corpse of the Pictish girl who screamed, and the Doctor realizes that it's getting stronger. The sunlight feeds it, making the beast stronger and the world darker. Kar blames herself and says that she needs to make it right, and the Doctor figures that she let the Eater through in the hopes that both sides would eliminate the other. He tells her that to save her village, she doomed the whole world.
Bill insists that the Doctor can save them, but the soldiers don't believe he can make a difference. She tells them that they're not cowards but scared, and scared is fine. Lucius agrees that they need a plan, and he's the commander because he's the oldest one left. He admits that he's only 18, and Bill tells them that she's going up to find her friend. She can't promise that they'll live, but she can promise that they won't die in a hole in the ground.
At the village, the Doctor says that they have to drive Eater back through the Gate. He wonders why it only opens when sunlight hits it, and Ban says that their ancestors built the Gate to control it. Kar says that she's remembering the dead, the ones who died because of her, and the Doctor tells her that every moment she does so, more living will joint hem. He tells her that it's time to grow up and fight her fight. The Doctor finds a lens and Ban explains that it poisons the light that the Eater consumes. The Time Lord tells them that they need more of them because it has optical cancellation properties, and they have to lure it back through the Gate before sunlight. Kar admits that she's still afraid, and the Doctor tells her that she has to face her beast anyway. She says that she can do it.
Bill and the centurions leave the cave and they come to a ladder leading up to the village. They hear the Eater in the darkness, and Cornelius panics and runs for the ladder. The Eater kills him and the other centurions climb up the ladder and emerge into the hut where the Doctor and the others are. Bill and Lucius are the last ones op, and the Doctor closes the trapdoor just in time. The Eater falls back to feed on the sunlight in the morning and grow to its full strength, and the Pictish and the soldiers prepare to fight. The Doctor tells them to stay back, and Bill tells them that there's no time for fighting. Kar says that they didn't want to fight until the Romans came and lay waste to everything that they love. Bill realizes that the Doctor is translating everyone's words, and the Doctor says that they're all children. He tells them that they can carry on slaughtering each other until no one is left, or they grow up. The Eaters are locusts looking for cracks between world looking for dimensions of light. Once they come through, they will eat the sun and the stars, and will keep eating there are no stars left. He tells them that he has a plan.
The two sides join forces, and the Doctor tells them that the Eater homes in on light. They'll use sound to lure the Eater to the cairn and then drive it back through the Gate. The creatures can only come through the Gates one at a time, and each Pict fought one for 60-70 years. The Picts start playing music, and Nardole goes outside. A crow caws words and then flies off as the Eater approaches.
The Eater runs into the cairn and the two sides use the lenses to train the corrupted light on it. The Doctor says that they have to hold it there until sunrise. The sun rises and the Doctor uses a lens to force the Eater back into the Gate. He tells the others that it will only stay there as long as the sunlight shines on the Gate, and asks for a weapon. When Bill wonders what he's doing, the Doctor explains that the Gate has to stay guarded, but human lifespans aren't long enough. He goes on for ages and can hold back dying until the sun goes out. Bill tells him that it isn't his job, and the Doctor says that it is. He explains that the TARDIS will take her home.
Kar says that it's time for her to grow up and fight her fight. When the Doctor says that she can't, Lucius tells her that he'll fight at her side. The Doctor warns that they will only last two seconds and then the solar system will be devoured, and Kar orders her people to stop the Doctor. Lucius says that all that remains of the Ninth legion will guard the Gate. He tells Kar that he will put her name in the stone and it will live forever, and they hug. Bill tells the Doctor that it's their destiny, not his, and Kar and the centurions advance into the gate. As he goes, Lucius looks back at Bill and smiles. The cairn shakes and Nardole says that with so many going through at once, the place is unstable. Everyone gets out of the cairn.
Later, everyone gather sin the field and Bill comforts Ban. He tells the crows to remember Kar's name. Nardole tells the Doctor that the crows are remembering Kar's name, and the Doctor admits to Bill that he was wrong. He admits that the Ninth Legion has always been there, holding back the dark. As they go into the TARDIS, Bill says that she thinks she can hear the music from the cairn. The Doctor merely says that music is funny like that.
Missy is in the TARDIS reading when the others return. The Doctor tells Nardole that Missy is doing some work for him, and that Missy is doing maintenance for him. Missy tells Bill and Nardole that she's biolocked out of the controls and can't go through the doors. The Doctor points out that Missy got them back from Mars, and she watched what happened on the monitors. When she says that Lucius and the others are trapped in the cairn, fighting forever with music, the Doctor tells her that they're not trapped. They're more than just fighting, and there's more than music. He's trying to teach her that she's trying to grasp the music but never hears the music.
In the present, Judy hears the music from the stones.
In the TARDIS, Missy listens to the music and considers. She cries as the Doctor watches, and wonders why. He suggests that she's trying to impress him, and Missy agrees. The alternative is that it's real and it's time for them to be friends again. Missy holds up her hands, and after the moment the Doctor says the problem with hope is that it's hard to resist.
Written by Gadfly on Jun 18, 2017