This episode first aired on December 2, 2018. Written by Ed Hime. Guest-stars Ellie Wallwork and Kevin Eldon.
The Doctor and her friends land on the edge of a fjord in northern Norway, where the only sign of life is a cottage that doesn't have smoke coming from the chimney, despite it being winter. Upon arriving at the cottage, the Doctor and company find that the cottage has been boarded up and Ryan spots someone inside. Entering, they find signs of life, with shoes scattered around the entryway and candy wrappers on the kitchen table. Searching upstairs, Ryan and Graham find a girl hiding in a wardrobe. A sandwich Graham has been carrying in case he gets hungry proves to be the ticket to persuade her to come out. Downstairs, the girl asks how they got in, and the Doctor says they were in the area and asks what the girl is afraid of. She explains that her father, Erik, went missing four days ago, taken by a monster living in the woods. The fact that the girl, Hanne, is blind only makes her situation harder.
As everyone looks around outside, loud roars are heard, which Hanne explains is the creature coming, as it hunts at this time every day. Searching the shed, Ryan and Yaz find bear traps, but the loud roars of the creature prompt everyone back to the cabin in a rush. Graham, sent upstairs to look out the window for signs of the creature, is distracted when he discovers the mirror at the other end of the room, Erik's bedroom, isn't reflecting him. Ryan and the Doctor come up to see it too, and the Doctor, after taking a peek, deduces that the mirror is a portal of some kind. Hanne wants to come through to look for her father, but the Doctor won't let her, telling her to stay with Ryan while she, Graham, and Yaz investigate. Hanne isn't happy, disliking Ryan for his automatic assumption that her father ran off instead of having been abducted. The Doctor writes a message to Ryan on the wall, suggesting that he assume Erik is dead, but says aloud that it's a map of the house's vulnerable points. On the other side of the portal, the Doctor and company discover a dark, maze-like space, which turns out to be an antizone, a space between the two halves of the portal, created only at points where time and space is in serious danger.
The Doctor, Graham and Yaz meet a mysterious figure in the antizone, who introduces himself as Ribbons and has the only light source. He offers to show them the path Erik took in exchange for the sonic screwdriver, and the Doctor insists on payment after delivery. Ribbons eventually tries to double-cross them, having cut the string the Doctor was using to mark their path, but gets eaten by the dangerous flesh-moths inhabiting the area. Fleeing, the Doctor, Graham, and Yaz discover the other end of the portal and a mirror-reversed version of the cabin. Downstairs, they find Erik, who's living with a mysteriously alive version of his late wife, Trine. And there's another new arrival: the late Grace O'Brien. Back in the normal reality, Ryan discovers that the monster roars are being played by a set of speakers outside, but Hanne, already suspecting that he's lying about something, knocks him out by slamming a door in his face, takes the key to her father's bedroom and heads through the portal herself.
Talking the situation out with Yaz, the Doctor realizes that the reality they are in is a Solitract, a sentient universe that was exiled from the main universe because the universe couldn't hold together otherwise. She also realizes that the whole situation is a trap and that they have to get Graham and Erik out of there. Graham, talking with Grace, is initially sceptical that she's really her, but starts to believe otherwise after a long enough talk. However, the Doctor arrives and insists they have to leave. Ryan, after coming to, eventually catches up to Hanne in the antizone, and they head to the other side of the portal. However, they attract the attention of the flesh-moths. Hanne makes it through, but Ryan is left behind to hide. In the Solitract, Hanne immediately realizes that this version of Trine isn't her mother, and Graham finally realizes that Grace isn't really Grace, as she doesn't care about Ryan's safety. Eventually, the Doctor persuades the Solitract to send everyone out except her, and the humans flee back to the normal universe. The Solitract, taking the form of a frog with Grace's voice, talks to the Doctor in a White Void Room. However, the reality is still collapsing because of the Doctor's presence, and the Doctor persuades the Solitract to send her back as well by explaining that friends try to help each other.
When everyone is back in the normal reality, Erik and Hanne decide to return to Oslo instead of their lonely cabin. The Doctor and her friends head back to the TARDIS to travel on, and Ryan finally calls Graham "Granddad" after they have a heart-to-heart about their grief for Grace.
- Alternate Universe: Lampshaded by Yaz, who initially assumes they've crossed into a parallel universe where Erik's wife and Grace are still alive. Turns out they are in the Solitract, a sentient universe that has been separated from the main universe since it was created and become the subject of Time Lord bedtime stories.
- Animal Motif: Frogs. Grace has a liking towards frogs and treasures a frog necklace that Graham gave her. In the ending, the Solitract takes the form of a talking frog with Grace's voice.
- Ask a Stupid Question...:Yaz: So why did you make the bear traps?
Erik: Because there are bears?
- Bad Liar: Hanne suspects the Doctor didn't scrawl a map on her wall when Ryan fumbles through an answer to a question about where the house is most vulnerable, which is supposedly what the map is for. Writing doesn't sound like drawing, so Hanne was already suspicious and Ryan's bad lying confirmed it.
- Big Creepy-Crawlies: The antizone is filled with rat-sized, flesh-eating moths.
- Blank White Void: When the Doctor is left the only visitor in the Solitract's universe, it drops the fake Norway for an appearance like this, leaving only a few beams mirroring those of the upstairs of the cottage, and a chair with the Solitract, in the form of a frog, sitting on it.
- Brandishment Bluff: When Ribbons puts a knife to Graham's throat, the Doctor gets him to back off by pointing her sonic screwdriver at him like a weapon. This is probably why Ribbons took the risk of trying to grab it later, to defend against the flesh moths.
- Break His Heart to Save Him: Done with a twist when the Doctor entices the Solitract with her far greater range of experiences compared to Erik, noting that the Solitract can only keep one of them. When the Solitract chooses her, Erik is convinced that it's not really his wife and is ejected.
- Brick Joke: In the first scene, the Doctor sees a sheep in the woods and mentions the Woolly Rebellion. In the last scene, noting that the sheep has gone, she suggests that it's probably off plotting.
- Once again, Team TARDIS open a cupboard with trepidation, only to find it doesn't hold something nasty or dangerous.
- To "Warriors' Gate", which also contains mirror portals in a collapsing labyrinthine Void Between the Worlds, though in this case the Blank White Void is on the other side of one of the portals, rather than the void itself.
- In "The Rings of Ahkaten", the climax had the Eleventh Doctor offer his memories to the parasite planet in hopes of appeasing it since no one else around has as many memories as he does. Here, the Thirteenth Doctor offers to stay with the Solitract because just having her, with all of her experiences, around will give it much more to work with than Erik would, which helps Erik realize his "wife" is an illusion. In both cases, the Doctor's solution doesn't work out, but there's an alternative — the Eleventh Doctor's past can't compare to the infinite possible futures that Clara's leaf offers the parasite, and the Thirteen Doctor can't stay with the Solitract after all because both universes will collapse, but she can promise to remember it as a friend and vice versa, which it accepts as enough.
- Pointed out by the AV Club review: As in "Last Christmas", a companion of the Doctor is tempted to stay in a false reality in which a deceased loved one is still alive. The main difference is that the Solitract doesn't intend to harm Graham, whereas the Dream Crab was slowly killing Clara.
- Continuity Nod:
- Two to "Rosa":
- Graham has started carrying a sandwich with him at all times, since they don't often stop to eat, something he picked up on during the previous episode.
- Graham tells the Solitract version of Grace that he met Rosa Parks.
- The Doctor says that her fifth grandmother (of seven) thought that her second grandmother was a Zygon spy.
- Two to "Rosa":
- Cosmic Flaw: The Solitract and the universe cannot coexist; The Universe ejected the Solitract from itself when it was forming so it could finish doing that and the Solitract has to be convinced to be alone again because the presence of matter from the universe is destabilizing it. The Antizone exists spontaneously as a buffer because the two cannot be in any kind of direct contact.
- Death by Materialism: Ribbons is killed when he tries to steal the Doctor's sonic screwdriver during a flesh moth swarm, his movement provoking them into eating him.
- Don't Go in the Woods: Invoked. The monster heard roaring in the forest surrounding the cabin is really just a speaker set up by Erik to keep Hanne from going in the woods.
- Eaten Alive: Ribbons, after trying to run from the flesh moth swarm.
- Eat Dirt, Cheap: The Doctor eats the some dirt and somehow deduces from this that the TARDIS has landed in northern Norway. She also deduces that, 25 miles away from their current location, there's an alpaca farm with a gift shop and bad Tripadvisor reviews. Her companions take this in stride, but politely turn down the dirt when she offers it.
- Eldritch Location: The antizone between the main universe and the Solitract, described by the Doctor as a thing created when time and space is under terrible threat as a means of protection. Extra points for containing an ecosystem and a person, being a (very small) world in its own right.
- Explain, Explain... Oh, Crap!: "When you put it that way, it sounds like a trap." *cue the Doctor and Yaz running frantically to find Graham*
- When the roars of the creature are first heard, Hanne says that it hunts at the same time every day. It turns out that it's actually a recording, set to play on a loop.
- When Graham tries to find out if the Alternate Universe Grace is real, he asks her to tell him about the frog necklace — she tells him exactly everything about it, including that he's wearing it to keep her close. While this initially seems like something she's intuited, it's actually because everything about this version of Grace comes from Graham's own mind.
- A Form You Are Comfortable With: The Solitract manifests before the Doctor in the form of a talking frog, telling her that it finds the form delightful as it once delighted Grace.
- Genius Loci: The Solitract is an Alternate Universe with its own sense of self-awareness.
- I'm a Humanitarian: Ribbons of the Seven Stomachs. He suggests having Yaz for lunch and Graham for tea, would like to snack on the Doctor's brain because she's so smart, and his knives are made from bones.
- I Just Want to Have Friends: Ultimately, the Solitract's motivation, as it's lonely because it can't be a part of the universe.
- It Can Think: When the sonic screwdriver doesn't work on the portal the second time, the Doctor realizes it's learning and adapting.
- Jerkass Has a Point: Ryan's first instinct, based on his own past, is that Hanne's father has deliberately run off instead of being taken by a monster. He's quickly scolded for being rude, but it turns out that he's partly correct; although Erik isn't fully aware of how much time has passed since he left Hanne, he has run off to a parallel world where a version of his wife exists, and the monster is just a recording he set up to keep Hanne in the house until he gets back.
- Jerkass Realization: When Erik returns and sees the Doctor's writing on the wall, he realises what he almost did and whom he nearly abandoned.
- Jump Scare: Several of these, such as Ryan being startled by the birds left to hang in the larder, and Erik when Team TARDIS unexpectedly appear in his universe.
- Lotus-Eater Machine: The Solitract creates illusions of people's dead loved ones in order to convince them not to return to the main universe.
- Madness Mantra: Hanne repeats the phrase, "it takes you away", when she's scared of the creature.
- Magic Mirror: The mirror in Erik's bedroom acts as a portal to an antizone between the main universe and the Solitract. In addition, the Solitract's universe is literally mirror-reversed, with the dock Graham speaks to Grace on flipped opposite to the way the one in the real world is oriented. Everyone's appearances are also flipped, with the Doctor's Fashionable Asymmetry making hers the most obvious change. Breaking the mirror destroys the portal.
- Meaningful Name: Solitract sounds similar to solitude, and it's fated to be alone forever since it can't be a part of the universe, as prolonged contact with any part of the normal universe will destroy it.
- Missing Reflection: When the portal is active, it doesn't reflect anyone standing in front of it. Graham approaches the mirror mystified after noticing this. When Ryan comes up and sees it, he asks if they would know if they were vampires.
- Moth Menace: The antizone is home to creatures called flesh moths that will strip the flesh from any source of meat they encounter.
- Non-Malicious Monster: The Solitract only wants to have company. It's too bad this threatens the universe's existence. Likewise the flesh moths, which are just animals out for a meal.
- Noodle Incident: The Doctor, upon seeing a sheep roaming the woods, mentions the Woolly Rebellion of 2211, which is a "bloodbath" that results in a renegotiation of the relationship between humans and sheep.
- Out-of-Character Alert: Though Grace seemingly has all the memories of the original, in truth she only seems to know as much as Graham himself, she reveals herself as a fake when she tries to talk Graham out of helping Ryan, which the real Grace would make her first priority.
- Parental Neglect: Erik isn't the father of the year. He saw his dead wife in another dimension and never wanted to leave. Not wanting his daughter to panic, he set up speakers to play sounds to keep his blind daughter in fear of leaving the house.
- Properly Paranoid: When the lantern goes out and Ribbons advises everyone to run, the Doctor tells her companions to remain still as Ribbons has demonstrated that he isn't to be trusted. Sure enough, it's shown that the moths would have eaten them if they had run and made themselves targets, so Ribbons gets eaten, instead.
- Reality-Breaking Paradox: The Solitract was exiled from the universe because its very existence runs counter to the natural order of the universe. The Solitract can be whatever it wants, in defiance of any and all natural laws, so the universe couldn't exist until the Solitract ceased to be a part of it. Likewise, having main universe humans in itself causes the Solitract to break down.
- Red Herring: Ryan and Yaz find several bear traps in the shed, which they assume to be proof of the mysterious monster in the woods. As it turns out, there is no monster in the woods, and the traps are there solely to deal with actual bears.
- Reverse the Polarity: Suggested to the Doctor by Yaz. The Doctor is delighted to hear it.
- Riddle for the Ages: Who Ribbons is, where he came from and how he ended up in the antizone are all mysteries, since he refuses to explain anything without payment prior to his death.
- Seen It All: Invoked by the Doctor to convince the Solitract to Take Me Instead. If it wants a friend, why not someone who's seen the entire universe it's pining after?
- "Scooby-Doo" Hoax: The creature roars that Hanne is afraid of are coming from a speaker, set up by Erik to stop her from leaving the cabin.
- Sdrawkcab Name: The text on Erik's Slayer shirt is reversed, which is a side-effect of crossing over to the other universe. When he returns to his own universe, the text becomes non-backwards again.
- Something Only They Would Say: While trying to find out if the Grace in the Alternate Universe is real, Graham asks her to tell him about the frog necklace he's wearing. She remembers it perfectly, making Graham that much more reluctant to believe she's a fake.
- Tae Kwon Door: Hanne disorientates Ryan at one point by slamming a door into his head.
- Title Drop: When Hanne is hiding under the table, all she can say is, "It takes you away."
- Tomato in the Mirror: Parodied. When Graham and Ryan first see the mirror without their reflections in it, Ryan says, "We'd know if we were vampires, right?"
- Tragic Keepsake: Graham has been wearing a frog necklace he gave to Grace for Christmas one year.
- Void Between the Worlds: The antizone, a maze of dark caves and Moth Menace swarms which extends the passage through the Magic Mirror portal because the universe can't be in direct contact to where it leads.
- Woobie, Destroyer of Worlds: The Solitract causes the fundamental laws of our universe to fail, meaning that it has to exist apart from our universe in complete solitude.
- You Have GOT to Be Kidding Me!: Ryan's reaction when he discovers the speaker outside that's playing the recording of the monster.
- You No Take Candle: Most of Ribbons' speech sounds like this, though he does have a few eloquent moments.
- Your Eyes Can Deceive You: Hanne can tell instantly that Trine isn't her real mother, unlike her father, who is unwilling to give up the illusion.