DOCTOR WHO IN DETAIL

In Scotland at The Devil's Cairn a young girl - Judy - puts her ear to the ground to hear music, despite her brother warning her that doing so would release ghosts that would eat her.

The TARDIS materialises at the same site in Aberdeen, but in 2AD. Nardole is wondering why Scotland is more important to the Doctor than guarding the vault. Bill has said she knows more about the Ninth Roman Legion than the Doctor, and he insists she doesn't. They split up with Bill going to search for the Ninth Legion that should be following the river, and the Doctor and Nardole going off to look for their last battlefield.

In a forest Bill finds a woman, Kar, by a fire remembering her parents. When Kar notices Bill she screams, draws her sword, and chases her. Whilst trying to get away Bill falls through a hall into a cave where she finds a Roman soldier. Bill is amazed she can talk to the soldier and realises the TARDIS is translating, and providing lip-sync.

Above ground the Doctor and Nardole are not having much look finding the 5,000 soldiers of the Ninth Legion. Nardole insists a crow is talking, and the Doctor says that all crows talk. It's just that in the future people have stopped listening so they are all sulking about it. The Doctor is more interested in a Pict which the local believe are doors between worlds, and close by is a dead Roman soldier whose bones have disintegrated. Looking further afield the Doctor and Nardole see the bodies of the Ninth Legion. The Doctor wants to find and warn Bill, but is stopped by a group of Scottish Barbarians.

As night falls Bill and the soldier climb out of the cave. The soldier tells Bill there is a safe location where a few survivors an to. In the forest the can hear growling, and the soldier is taken by a monster as Bill runs. She finds the survivors and is helped inside as the monster attacks her. The soldiers manage to block the entrance enough to stop the creature.

The Barbarians take the Doctor and Nardole to one of their settlement to wait for Kar - the keeper of the gate. Even though they have swords pointed at their necks Nardole tries to ingratiate himself by offering to make the Barbarians popcorn. The Doctor takes a different tactic and demands to know what killed the Roman soldiers. Kar enters and says, as the gatekeeper, it was her. Whilst Kar is giving a speech about the Romans the Doctor throws Nardole's popcorn into a fire. When it explodes the Barbarians are startled allowing the Doctor and Nardole to escape.

As Kar is a gatekeeper, and the Barbarians kept talking about gates, the Doctor begins to believe the Cairn really could be a door to another world. As he enters wall opens to reveal a portal to another world full of strange creatures. The Doctor quickly leaves the Cairn and finds Nardole outside talking to the Barbarians. Nardole is surprised to see him as he has been gone for two days. The Doctor confronts Kar about the portal. She tells him that in every generation a warrior travels through to stop creatures, called Eaters of Light, from crossing. Kar, as the gatekeeper, should have stopped the creature. With a Roman Legion close by Kar let one through thinking the Roman Legion would kill the creature.

In the settlement the Doctor learns they have a strange glass that has optical cancellation properties that the creatures don't like. The Doctor comes up with a plan to drive the creature back through the portal whilst it is still weak. This has to be done before sunrise before the creature feeds and becomes stronger.

After spending time with the Romans Bill has gained their trust and tells the about the Doctor. Using underground passageways they make their way to the settlement. As they reach it their torchlight attracts the creature. The Doctor helps Bill and some of the Romans into the settlement, but a few are taken by the creature. Whilst Bill and the Doctor block the trap door to stop the creature the Romans and the Barbarians are circling the room, swords drawn.

For once, using the TARDIS telepathic field, the Romans and the Scottish can finally talk to, and understand, each other. The Doctor is able to convince them to fight together to stop the creatures from entering this dimension and eating all light - starting with the sun and then all stars. They all go to the Pict and set fires to attract the creature. Using lenses made from the strange glass the creature is driven back through the portal.

The Doctor tells Bill that there needs to be someone on the other side to stop the creatures from getting back through. The problem is that human lifespans are tiny, so there is only really one option. Instead Kar, and several Roman soldiers, go through the portal. So many people going through at one time makes the whole area unstable and the portal is covered. Outside the last remaining Roman soldier tells a crow to remember the name Kar.

Bill tells the Doctor he can't fight every battle, and that he has a vault to guard. Inside the TARDIS Nardole is annoyed when Missy appears. She should be in the vault.