Montegue = Montague
A young Walter Simeon is alone, making a snowman on his front yard as the other children play together. Walter's mother comes out and suggests that he play with the other children, but he refuses, insisting that he doesn't need anyone. His mother goes back inside and Walter tells the snowman that the other children are silly. The snowman talks back to him, saying that Walter is right. When Walter starts to run away, the snowman says that he doesn't need anyone else. Intrigued, the boy goes back and the snowman says that it can help him.
Fifty Years Later
A now elderly Walter Simeon has workers taking samples from snowmen around the Great Intelligence Institute that he owns and supervises. He then takes the jars of snow samples into the main office of the Institute and scrapes them into a large glass globe filled with snow. As he works, the Voice asks him if everything is ready. Simeon assures it that it is and it asks if the workmen will be a problem. The old man assures him that he promised to feed "them." Simeon then goes outside and the workers wonder when he will feed them as promises. As they look up, Simeon says that he didn't say who he would feed them to. The snowmen in the yard come to live, bear enormous fangs of icicles, and advance on the screaming workmen.
At the Rose & Crown Tavern two days before Christmas, Clara is working as a barmaid and goes out into the back alley. She glances away for a moment and then looks back to discover that a snowman is suddenly standing in the alleyway. A man brushes past Clara and she asks him if he made the snowman. The Doctor denies it but then examines the snowman carefully. He tells Clara that it's made of snow that once remembers being a snowman but the barmaid finds the idea ridiculous. When she introduces herself, the Doctor loses what little interest he had and tells her to keep the snowman. He goes to his carriage and Clara, intrigued, runs after it.
Inside the carriage, the Doctor's Silurian friend Vastra contacts him via the speaker tube and points out that he's finally taking an interest in someone else. The Doctor says that Clara means nothing to him and that she can't even find him because he didn't even mention that he was the Doctor. Clara bursts in through the roof hatch and, overhearing him, asks, "doctor who?"
Elsewhere in London, Captain Latimer returns home and finds Simeon waiting for him. Simeon points out the manor's pond and comments that Latimer's governess drowned there a year ago on Christmas. The ice froze over her and her corpse wasn't discovered until a month later. Latimer wonders what Simeon wants and the old man tells him that what is now in the pond belongs to the Institute. He gives Latimer his card and walks away. As Latimer goes inside, the ice in the pond begins to crack.
As Simeon walks back to the Institute, a veiled Vastra and her wife, Jenny, cut him off and ask about the Institute. Simeon knows that Vastra is the famous "Veiled Detective" of London and unveils her. He doesn't react to her Silurian features, and Vastra points out that the snow in the alley and elsewhere is exuding a low-level telepathic field. She notes that it absorbs the memories of those nearby and could be a weapon in the wrong hands, and Simeon warns her that the last winter of humankind is coming. He dismisses Vastra, saying she can do nothing about the snow, and starts to walk away, and the Silurian says that she knows a man who can do something. Simeon merely says that he looks forward to meeting that man and leaves. Once he's gone, Jenny warns her wife that the Doctor won't help and Vastra admits that if the Doctor won't take an interest then they need a miracle.
Strax the Sontaran, wearing a Victorian butler's uniform, accompanies the Doctor as they follow Simeon and watch him collect snow from snowmen scattered throughout the city. The Doctor realizes that the "snow" is alien in origin and suspects that Simeon is trying to make a profit from it, following traditional Victorian values. However, at that point the Doctor dismisses the entire matter, ignoring Strax's objections to his apathy and pointing out that the universe doesn't care. They go back to the carriage where the Doctor has locked Clara up and he tells her that he plans to use a memory worm on her to erase the last hour of her memories by touching it to her bare skin. Strax goes to get the worm but forgets the protective gloves and forgets what he was doing and who Clara is. The Sontaran crawls beneath the carriage and grabs the memory worm again, only to forget what he's doing, while Clara hands the Doctor the protective gauntlets.
The Doctor finally gets the worm into a jar, warning Clara that if it actually bites someone then they could lose decades of their memories. When he asks Clara why she hasn't run, she says that she will once he explains how the snowman in the alleyway built itself. As she mentions the incident, a snowman appears nearby in the street. The Doctor tells her that the snow responds to the thoughts of those nearby, and Clara points out the snowman to him. As she keeps thinking of snowmen, more and more of the fanged creatures build themselves out of the snow, cutting off their escape. As the creatures close in, the Doctor tells Clara to imagine them melting and concentrate... and they explode into water.
As Clara dries off, she wonders what to do if she thinks of snowmen again. When the Doctor points out that she knows what to do, she reminds him that she can't do it if he makes her forget what happened. The Doctor puts Clara in the carriage and sends her on her way, saying that the snow isn't her problem. As Strax drives the carriage away, he's unaware that Clara has slipped out. She follows the Doctor to a deserted street in the heart of London and watches as he looks around, whistles innocently, and then leaps up and grabs an invisible ladder. Once he pulls it down, he climbs up the ladder and disappears, pulling the ladder after him.
Clara goes over and finds the ladder, jumping up. The ladder leads to a spiral staircase leading up into the clouds and Clara realizes that no one can see her on the staircase. She walks up the stairs and finds herself on what appears to be a solid cloud bank. The TARDIS is nearby and she knocks on the door, but then ducks out of sight as the Doctor looks out. He circles the TARDIS and Clara circles opposite him and then runs down the stairs. The Doctor finds her discarded shawl, sniffs at it, and looks down the staircase.
Simeon returns to the Institute with more snow samples. As he puts them into the sphere, the Voice tells him that it will snow again the next day, making it stronger. Once it has enough strength, it tells Simeon that the "drowned woman" and "the dreaming child" will give it form.
The next morning, Clara wakes up, grabs a carpet bag, and goes outside. Her Uncle Tom, who runs the Rose & Crown, asks her to stay but Clara tells him that he can manage now that his wife has recovered. When he wonders where she's going, Clara says that he'd never believe him. She takes a carriage to the Latimer estate, donning a governess' dress from her bag on the way there. The maid, Alice greets Clara as "Miss Montague" and welcomes her back. They discuss Christmas the next day and how the captain's children, Digby and Francesca, are looking forward to it. As Clara enters the manor, Latimer summons her to his study and tells her that Francesca has been having nightmares. He wants the governess to discuss them with Francesca, admitting that family issues aren't something he's comfortable handling. He inadvertently describes Clara as "pretty" and then hastily corrects himself.
Clara finds the children playing outside and they tell her what they've been doing. She then talks to Francesca and the girl explains that she's been having nightmares about their former governess, Clara's predecessor. Clara suggests that Francesca talks to her father but Francesca clearly doesn't want to. Digby takes Clara to the pond and shows her where the previous governess died, and admits that he hated her because she was always cross. Clara notices that the pond is still frozen over and remembers the Doctor telling her about how the snow feeds on the thoughts of those nearby. She asks Francesca about her nightmares and the girl explains that the governess threatens to come out of the pond and come after her on Christmas Eve. Digby, overhearing, says that his sister needs "a doctor."
Later, Clara goes to find the invisible ladder in the street. As people stare, Clara first calls for the Doctor and then tries to jump up and grab the ladder. Jenny runs over and tells Clara that they don't want to attract attention and takes the governess to Vastra's home. Clara is shocked to see Vastra's alien features but quickly overcomes her surprise. Vastra sits her down and tells her that she is only to answer her questions with one-word answers. Clara explains that she needs "help" and that she hopes the Doctor will help her out of "kindness" The Silurian warns her that the Doctor is no longer kind and doesn't help people, even though he once did before he suffered a loss. Vastra informs Clara that neither she nor Jenny approve of the Doctor's new attitude but support him out of friendship. When Clara wonders what to do next, Vastra says that she must give her a one-word answer that will convince the Doctor to help her.
A few minutes later, Vastra calls the Doctor in the TARDIS. She passes on Clara's one-word plea for help: "pond."
At the institute, the Voice detects an intelligence more powerful than anything in that time or space. The Doctor suddenly bursts in, dressed as Sherlock Holmes, but Simeon points out that Sherlock Holmes is a fictional character. Undeterred, the Doctor strikes the globe with his cane and Simeon grabs him. The Intelligence speaks up, introducing itself and saying that the Doctor is not of the planet Earth. Simeon rings for help but the Doctor casually seals the door with his sonic screwdriver and examines the globe. He realizes that the Intelligence needs human DNA captured in ice so that it can make a body for itself. Looking around, the Doctor spots a fold of newspaper clippings and throws it on the floor. It opens to the spot that Simeon has been studying: a news article about the governess' death at Latimer's manor. Simeon's men finally smash open the door but the Doctor slips out through the window.
At the Latimer estate, the Doctor scans the pond and realizes that the ice is sentient and was able to examine the governess when her body was trapped beneath the ice for several weeks. Strax arrives and offers to help the investigation, and the Doctor insists that he isn't investigating anything. Clara waves to the Doctor from the window and gestures him up, and he hesitantly responds, going inside. As he leaves, the ice in the pond starts cracking open.
Outside, Vastra, Jenny, and Strax watch the manor and see Simeon pull up in his carriage.
As she waits for the Doctor, Clara tells Francesca that the Doctor lives on a cloud in the sky and is coming to help them. The door opens and Clara assumes that it's the Doctor. The children scream and Clara turns and discovers that it's a walking ice statue of the dead governess. She is very cross at them and advances on them. Clara gets the children out of the room and into the adjoining playroom, locking the door behind them. As the Ice Governess starts smashing through, Clara tells Francesca not to think about the creature, but the girl is too scared to think of anything else.
As the Ice Governess breaks through the door, Digby wonders where the Doctor is. A puppet in the toy theater suddenly springs up and fires the sonic screwdriver at the Ice Governess, destroying it. The Doctor emerges from the theater, smiling.
At the carriage, Simeon activates a device that sprays the sentient snow over the manor.
The Doctor scans the remains of the Ice Governess, which have soaked into the carpet, while insisting that he doesn't fight monsters anymore. However, he sees himself in the mirror and realizes that he's put his bowtie back on. Ice forms on the window and the temperature in the room drops, and the Ice Governess reforms out of the carpet. The Doctor tells everyone to run down the stairs and they find Latimer, who demands to know what is going on. Caught off-guard, the Doctor blithely says that he's Clara's girlfriend and they were upstairs kissing. Latimer is shocked but before he can pursue the matter, Alice runs in and warns them that snowmen are building themselves in the garden. The doorbell rings: it's Vastra and Jenny, with snowmen behind them. Strax comes in and gives them a status report, and Alice faints in shock.
As the Doctor tires to work out what to do next, the Ice Governess comes down the stairs. Jenny throws a forcefield grenade, trapping it on the landing, and Strax determines that the study is the best location to build a barricade. As they go in, Vastra asks the Doctor if he's enjoying himself but he just smiles and tells her to shut up. Inside, Strax realizes that the snowmen are assuming a defensive formation and Vastra concludes that there is something in the house that they want. The Doctor realizes that they want the Ice Governess because she contains the dead governess' human DNA, encapsulated in ice form. To evolve, the sentient snow needs the DNA to build itself human forms and create an army made out of ice.
Someone rings the doorbell and the Doctor tells everyone that he'll deal with it. However, Clara insists on going with him, much to the Doctor's irritation. She figures that he likes her and kisses him The Doctor, surprised and shocked, blushes and runs to the door where Simeon and his snowmen army are waiting outside. Simeon gives the Doctor five minutes to turn the Ice Governess over to him and walks away.
The Doctor realizes that they have to get the Ice Governess away from Simeon and out of the house. He gives Clara an umbrella and then expands the forcefield, placing himself and Clara inside of it with the Ice Governess. They run past the monster and up to an outside window. When Clara gets her bustle caught on the windowsill, the Doctor pulls her through and tells her to take her clothes off. Realizing what he said, he hastily apologizes and Clara says that she understands. When Clara wonders what his plan is, the Doctor hands her the umbrella and tells her to tell him what his plan is. The Ice Governess turns into snow and starts blowing out through the window, while Clara goes over what has happened. She figures that they didn't go up to escape, and they're not moving so they can't be there to hide. Clara then reaches up and snags the invisible TARDIS ladder with the umbrella handle, pulling it down. The Doctor smile sin satisfaction, climbs up, and pulls Clara up after him.
As the Ice Governess climbs the TARDIS stairs after them, the Doctor asks Clara if she's a barmaid or a governess. She refuses to answer as they reach the top of the cloud level. The Doctor condenses the super-dense water vapor supporting the TARDIS, trapping the Ice Governess within the cloud bank. He then takes Clara into the TARDIS and explains that it travels through time and space... and it's his. She looks around and asks if it has a kitchen, commenting that she likes to make souffles. The Doctor remembers Oswin, the woman he never met face-to-face in the Dalek asylum, and how she liked to make souffles. He then asks Clara why she followed him from the bar. Rather than answer his question, Clara asks one of her own: why did he give her the umbrella when he was tall enough to reach the ladder on his own. She figures that he did it deliberately knowing she would follow him, and asks why she brought him along. The Doctor gives her a TARDIS key and says that he never knows why, only who.
The Doctor tells Clara that this is the day that everything begins for her, the Ice Governess breaks through the cloud bank, grabs Clara, and drags her outside. The Doctor follows them but is unable to do anything as the creature backs off the cloud bank and falls... taking Clara with it.
As they watch from the manor, Vastra and the others see Clara hit the ground. Using a scanner, the Silurian confirms that she's dead. They watch as the Doctor materializes the TARDIS around Clara's body and the Ice Governor's shards, and then rematerializes it inside of the manor. Strax uses his advanced equipment to try and revive Clara, while Vastra talks to the Doctor inside the TARDIS. He assures her that the ice shards are helpless as long as they're inside the TARDIS and angrily insists that Clara will be fine. Despite Vastra's warning that even Strax can't do miracles, the Doctor refuses to accept that the girl might die.
The Doctor rejoins the others and hands Jenny a map box of the London Underground, a box with the ice shards inside. Clara wakes up and the Doctor tells her that she'll live and gives her the TARDIS key. She asks if the Doctor will save the world from the snowmen and he says that he will if she comes with him. Clara agrees and the Doctor wishes her a merry Christmas. He then takes the box back from Jenny and goes outside. Simeon is eager to get the shards but the Doctor slams the door in his face and goes to the TARDIS. Vastra goes in with him and warns him that the universe doesn't make bargains. The Doctor insists that the universe owes him and dematerializes the Ship.
When Simeon returns to the Institute, he finds the Doctor and Vastra waiting for him. The Doctor points out that the Intelligence hasn't said anything to him or Vastra while Simeon was gone, and then holds up the box. The Intelligence speaks up, intrigued by the map markings on the box, but the Doctor uses his sonic screwdriver on it, scrambling its voice. He explains that he's stripping away its disguise and the Intelligence speaks in the voice of Walter Simeon as a young boy. Simeon waves and the Doctor tells him that the Intelligence is just a telepathic reflection of the old man's thoughts and fears. Because Simeon believes in the Victorian ideal of ice people, he's creating an army of ice people to conquer the world.
Simeon refuses to accept what the Doctor is saying and grabs the box from him. However, when he opens it, the memory worm from earlier leaps out and bites him on the hand. As it drains his adult memories, the Doctor says that the "Intelligence" is powerless without a host to give it thoughts. However, the Intelligence rallies and says that it isn't that easy. Outside, the alien snow falls across London and snowmen start appearing in huge numbers... and grow steadily larger.
The Doctor wonders how the Intelligence can survive with Simeon and it explains that it has outlived the dreamer. It then jumps into Simeon's body, gaining human form at last now that there is room in the old man's mind with his loss of memory. The Simeon/Intelligence easily smashes Vastra aside, grabs the Doctor, and begins freezing him solid with its touch.
As Clara's lifesigns fade despite Strax's best efforts, Latimer comes to her side. Clara takes his hand and tells him that his children need him to hold them. The captain admits that it isn't his area and Clara, crying, tells him that he has to now.
Across the London, the falling snow suddenly turns to water. The Simeon/Intelligence collapses and the Doctor realizes that the emotionally-sensitive snow is responding to something so strong that it's drowning out the Intelligence. He and Vastra go to the window and realize that the falling snow-turned-rain is tears, not rain. They return to the manor and find Latimer and the children crying over the dying Clara. Strax warns the Doctor that the girl only has a few moments left and the Doctor tells her that the two of them saved the world together. She tells him not to go back to being alone and the Doctor assures her that he won't, and Clara tells him that he's a clever boy and he should start running. The clock chimes midnight and Clara dies as Christmas Day dawns across London.
Later, the Doctor, Vastra, and Jenny watch from a distance as the Latimers attend Clara's funeral. Vastra wonders if the Intelligence poses any further danger and the Doctor vaguely remembers encountering a Great Intelligence and its snowmen in his past. The Latimers leave and the Doctor goes to Clara's grave. The tombstone says that her full name was Clara Oswin Oswald and the Doctor remembers how Oswin on the Dalek asylum told him the same things as she died that Clara did. He realizes that something impossible is going on and tells Jenny and Vastra to stay there while he leaves in the TARDIS to find Clara. As they watch him go, Vastra wonders if the universe made a bargain with the Doctor after all.
In the present, a woman that looks just like Oswin and Clara goes to the tombstone in the same cemetery. Her friend looks around nervously but Clara says that she doesn't believe in ghosts.