The Doctor and the Companions are in the woods of Norway. The Companions are admiring the scenery while The Doctor is tasting bits of forest floor plants to get her bearings.
Ryan spots a cottage a distance away. They go to inspect and find it boarded up but there are signs of someone inside. The Doctor uses her sonic to unlock the door's three locks.
Ryan and Graham investigate upstairs and find a child in a wardrobe, wearing sunglasses and clutching a bat. Graham convinces the child to come out with a sandwich he carries for emergencies. The girl is named Hanne and she is afraid of something that took her father. When The Doctor asks Hanne to describe what the thing looked like, they realize that she's blind. She asks for their help in finding her father.
Her father has been missing for four days. Her mother died earlier. Hanne warns them that the thing hunts at the same time every day.
Yaz and Ryan search the farm's shed and find dead game birds and a large number of animal traps. A roar is heard and Hanne shouts that the thing is coming. Ryan and Yaz show The Doctor one of the traps. Another roar is heard and they run for the house.
The Doctor finds Hannah under her table repeatedly saying, "It takes you away, it takes you away." She reassures her that they'll keep her safe.
Graham is upstairs keeping a lookout but he hears something and turns around and sees a mirror but no reflection. Ryan comes up behind him and startles him. He doesn't see any reflection either. Graham reaches forward towards the mirror and a noise starts up and the surface changes. The Doctor arrives suddenly and warns them to get away from the mirror. Suddenly the mirror is showing a reflection again.
They explain what they saw and then the noise is heard again and the reflections disappear. The Doctor points her sonic at it and jams up the function. She sticks her head through and discovers that it's a portal.
Yaz and Hanne join them and The Doctor explains that the mirror is a portal. She plans on taking another look and Graham and Yaz plan to join her. When Hanne says she wants to go, The Doctor says she doesn't want to risk her and plans to leave Ryan with her.
Before leaving Ryan and Hanne, The Doctor writes a chalk message on the wall to the Companions but tells Hanne that it's a map. The message is, "Assume her dad is dead. Keep her safe. Find out who else can take care of her."
Hanne tells The Doctor her father's name is Erik. The Doctor leads Yaz and Graham through the mirror.
On the other side, they are in a dark cave system. The Doctor ties a string to give them a path back. They see a light, move towards it and see someone sitting there. He says he has neither seen Erik nor the monster and he will not give them his lantern. When he sees The Doctor's sonic he suddenly remembers seeing Erik and is willing to trade his information and lantern for the sonic. His name is Ribbons.
Ryan and Hanne are talking but Hanne is upset with him for suggesting that her father might've abandoned her. She also figures out that The Doctor wrote a message not a map on the wall. She insists on going through the mirror but Ryan drags her out of the bedroom and locks the door.
The Doctor continues to question Ribbons but he is not forthcoming with information. They encounter a flesh moth which Ribbons distracts with a dead rat carcass. He gets the group to move ahead of him and he cuts The Doctor's string.
Back in the house, Ryan sees some wiring and follows it out of the house and discovers speakers which are making the roaring noises. When he comes in to tell Hanne, she knocks him out with a door to the head and steals the key.
Through the portal, Ribbon reveals that he's cut their string and demands the sonic or he will kill Graham. The flesh moth reappears and he says that more will come and the others should run. He tells them that they are in the anti-zone which The Doctor explains is a very bad thing. The Doctor realizes the moths are attracted to movement and warns everyone to stay still. They do and when Ribbons tries to grab the sonic, the moths attack him. The Doctor retrieves her sonic while the moths are feeding and they run, find the portal and jump through.
On the other side, The Doctor realizes they are in a different bedroom.
Hanne lets herself into her father's bedroom and enters the anti-zone. She finds The Doctor's string and follows it. Ryan regains consciousness and rushes into the anti-zone after her.
The Doctor, Yaz, and Graham find Erik in the alternate cottage. He explains that he rigged up a sound system to scare Hanne into staying in the house while he was gone. He tries to get them to leave but The Doctor notices two plates on the table. Trine, Erik's dead wife and Hanne's mother, comes in.
When they try to convince Erik to return to Hanne, Trine asks if they want to see their friend who arrived just before they did. She's in the backyard, humming a tune that Graham recognizes immediately. It's Grace.
Ryan catches up to Hanne and tells her about the animal recordings.
Grace and Graham talk and Graham tests her to see if she's really his Grace. She likes frogs and he's wearing a necklace that he bought her for Christmas two years ago. He still doesn't believe it.
Reasoning the situation out, The Doctor realizes that the anti-zone is a buffer between the dimensions because this mirror dimension is dangerous. She deduces that the mirror world is a creation of a sentient universe called the Solitract and that they're in the Solitract plane, a separate exiled existence. Now the Solitract has built a portal to lure people over.
In a hurry now to get Graham and Erik back to the real world, The Doctor can't reopen the portal. Yaz recommends a different frequency and it seems to work for a bit.
Inside the anti-zone, Ryan and Hanne are hiding from the flesh moths when The Doctor activates the portal. While Grace holds Graham back, Ryan tells Hanne to run for the portal and she emerges in the mirror world just as The Doctor loses control of the portal.
Erik hugs Hanne and tells her her mother is alive. When Trine hugs her, Hanne rejects her immediately. There's a sudden, violent tremor. She asks where Ryan is and Yaz realizes he must still be in the anti-zone. Graham insists The Doctor open the portal to save him but Grace protests.
The Doctor implores Trine and Grace to let them return to the real world because the mirror world is starting to fall apart because of the presence of too many real world people. When she questions why the Solitract would build a world to lure and trap people, she sees Grace reach for Graham's hand and figures out that the Solitract is lonely and wants interaction. Grace tries to convince Graham that she's real but Yaz tells her that she'll never be Grace. Trine retaliates by zapping her back through the portal.
Erik tries to convince Hanne that this world is safe. She tells him that he hasn't been well and then tells Trine that she hates her for pretending to be her mother and fooling her father. Trine zaps her through the portal too.
When Graham asks Grace if they should help Ryan in the anti-zone, she tries to tell him that Ryan will be fine on his own. Graham realizes then that this can't be Grace because the real one would always want to help Ryan. He calls her a fake and she zaps him through the portal and then disintegrates.
Despite everything he's seen, Erik still can't leave his Trine and The Doctor feigns resignation for a moment but then offers her longer existence and richer experiences in place of Erik. Trine considers this and Erik finally realizes that she isn't his wife. Trine zaps him through the portal and the mirror world dissolves. The Solitract manifests as a frog and they start discussing The Doctor's universe but The Doctor notices that the world is still destabilizing and convinces the Solitract that she has to return to her universe. She promises they will always be friends and the Solitract zaps her into the anti-zone.
The anti-zone is disintegrating but everyone makes it safely out the portal back into the real world and then the sonic smashes the mirror.
In the aftermath, Erik shares that he and Hanne will return to Oslo. Hanne thanks The Doctor and hugs Ryan. Ryan and Graham discuss Grace and Ryan calls him Granddad for the first time.