Young Walter is out in front of his home, making a snowman while the other children play with each other. Walter's mother comes out and suggests he play with the others, but Walter insists that he doesn't need anyone else. His mother worries that it's not healthy but walks away, resigned. Once he's alone, Walter tells his snowman that the other children are silly. The snowman echoes Walter's words, agreeing with him. The boy starts to run away but the snowman's voice echoes in his head, telling him that he doesn't need anyone else. After a moment, Walter goes back to the snowman, who says that it can help him.
Fifty Years Later
An older Walter Simeon has workmen take snow from snowmen and place them in glass jars marked "GI for the "Great Intelligence Institute." Simeon then takes them to the institute manor where his "voice" is inside of a sphere filled with moving snow. The Voice says that a great swarm is coming and asks if everything is ready. As Simeon places the snow into the sphere, he assures the Voice that everything is going according to plan. When the Voice asks if the workers will be a problem, Simeon says that they won't and that he promised to feed "them."
Simeon goes out on the balcony and looks down on the workers gathered in the yard. When the head workman asks when he will feed them, Simeon says that he didn't say who he would feed them to. The snowmen come to life and advance on the screaming workers.
At the Rose & Crown Tavern, a waitress, Clara, gathers the tankards from the customers and then goes out into the back alley. A man passes by as Clara notices a snowman standing in the alley where there wasn't one moments ago. She asks the man if he made it but he denies it, and the man--the Doctor--turns back and examines it. He suggests that it's snow that remembers once being a snowman but Clara dismisses the idea as silly. Clara introduces herself and the Doctor suggests that she keep the snowman and walks away. The waitress goes after him and asks where he's going, but the Doctor simply says, "those were the days" and keeps walking into the night. After a moment, Clara runs after him and sees the Doctor leave in a carriage.
In the carriage the Silurian Vastra addresses the Doctor through the speaking tube, saying that it's nice to see him taking an interest in someone. He dismisses Clara as inconsequential and says that she can't find him because she doesn't even know his name. Clara suddenly pops in through the roof hatch, overhears him mention his name "Doctor," and asks "Doctor who?"
When Captain Latimer arrives at his manor, he finds Dr. Simeon waiting for him. Simeon points out that Latimer's governess drowned in the pond in front of the manor. When the pond froze over, Latimer didn't find her until a month later when the ice melted. When the captain asks what he wants, Simeon informs him that what is growing in the pond belongs to his institute, presents his card, and walks away. In the pond, the ice begins to crack.
As Simeon walks down an alleyway, Jenny and a veiled Vastra come at him from both ends of the alley. They express an interest in his institute, and Simeon knows of Vastra as the famous "Veiled Detective." He steps forward and unveils Vastra, unsurprised to see that she's a Silurian. Undeterred, Vastra picks up some stray snow and says that it has a low-level telepathic field and can pick up the memories of the people around it. She points out that it could be a dangerous weapon and Simeon says that a terrible winter is coming: the last winter of humankind. He's confident that Vastra can do nothing about it and walks away. When Vastra says that she knows a man who can do something, Simeon says that he looks forward to meeting him and goes on his way. Jenny warns Vastra that the Doctor won't help them and the Silurian admits that they will need a miracle.
The Sontaran Strax, dressed in Victorian garb, is following Simeon and confirms that he's taking samples from snowmen across London. The Doctor is with him and realizes that the snow is alien and figures that they should look for a Victorian trying to make a profit. However, he isn't interested in pursuing the matter further despite Strax's objection to his apathy. As Clara tries to break out of the carriage where the Doctor has locked her up, the Doctor tells Strax that the one thing he's learned is that the universe doesn't care.
The Doctor goes to back to the carriage and assures Clara that they won't harm her. He does tell her that she'll forget they ever met and tells Strax to bring the worm. When Clara wonders what he means, the Doctor says that the worm will erase the last hour of her memory when it touches her bare skin. However, when Strax comes back, he has no memory of going to get the worm or of encountering Clara. The Sontaran climbs under the carriage to find it while Clara giggles, realizing what will happen next. Strax grabs the memory worm and promptly forgets about it again, while Clara finds the protective gauntlets in the carriage.
The Doctor puts on the gauntlets, grabs the worm, and crams it in a jar, explaining that a bite could cause decades of memories to fade. He's intrigued that Clara hasn't run and she tells him that she will once he explains how the snowman built itself. Another snowman assembles itself down the street as the Doctor explains that the snow responds to the thoughts of nearby people. Clara shows him the snowman and he realizes that she was thinking about it, and another snowman forms itself. When they try to run, another snowman cuts off their exit. More snowmen appear and the Doctor tells Clara that they're mirroring her thoughts. He tells her to picture them melting and she concentrates... and they turn into water.
Clara wonders what to do if the snowmen do the same thing again, and the Doctor says that now she knows what to do. She points out that she won't if she forgets, and the Doctor realizes he can't use the memory worm on her. He puts Clara in the carriage but tells her not to come looking for him. She asks if they should warn people about the snow but the Doctor says that it's not his problem and sends her on her way. Strax drives the carriage away, unaware that Clara has slipped out. She follows the Doctor to an empty street and watches as he jumps up and grabs a ladder hanging in mid-air. The Doctor pulls the ladder down and climbs up into nothingness and then pulls the ladder up after him.
Clara goes over and finally manages to grab the ladder and climb up. She finds herself on a spiral staircase leading up into the sky, invisible to all passersby. As the ladder pulls itself up, Clara ascends the stairs up into the clouds. The cloud bank is seemingly solid and standing nearby is the TARDIS. She walks over to it and knocks on the door, but then ducks around the side as the Doctor peers out. He calls out and then circles around to find out who knocked, and Clara circles around opposite him and runs down the stairs. The Doctor finds her discarded shawl and hears her footsteps going down.
At the Institute, the Voice tells Simeon that the snow will fall the next day, making it stronger. Then the drowned woman and the dreaming child will give it form.
The next morning, Clara wakes up in her bed, smiling. She packs a case and steps outside, and realizes that the snow has melted. Her Uncle Tom asks her to stay but Clara says that she has her own work to get back to. When he wonders what it is, Clara says that he'd never believe her. Once she's alone in a carriage, Clara dons a governess' uniform and goes to the Latimer manor, where they know her as Miss Montague. The maid, Alice, greets her and assures her that the children, Francesca and Digby, are looking forward to Christmas the next day. Latimer summons Clara to his study and explains that Francesca has been having nightmares. He wants Clara to talk to the girl, inadvertently describing her as "pretty" before correcting himself and saying "young."
Clara goes outside to where Digby and Francesca are playing. They run to greet her and she insists on proper manners first. Once they tell her what they've been doing, Clara talks to Francesca privately and the girl explains that she's been having dreams about their former governess. As Francesca talks to the governess, Latimer watches from the study window. Clara suggests that the girl talk to her father but Francesca is clearly uncomfortable doing so. Digby takes Clara to the pond and shows her where the governess died. He admits that he hated the governess because she was always cross. Clara notices that the pond hasn't thawed, and remembers the Doctor's warning about how the telepathic snow feeds on her thoughts. She asks Francesca what she dreams about and the girl says that in her nightmares, the governess is always cross with her and threatens to come out of the pond on Christmas Eve to punish her. Digby thinks that his sister has gone mad and says that she needs "a doctor."
Later, Clara goes to the street where she found the TARDIS ladder and calls to the Doctor. When she gets no response, Clara tries to grab the ladder as the passersby stare at her. Jenny runs over and says that they don't want to attract attention. When Clara realizes that she knows the Doctor, Jenny takes her to Vastra's home. Clara takes in Vastra's alien appearance while Jenny explains that Clara is to answer Vastra's questions with one-word answers. When Vastra asks why Clara came looking for the Doctor, the girl finally says one word: curiosity. She wants "help," and thinks the Doctor will help her because of "kindness." Vastra warns that the Doctor isn't kind and doesn't help people, but admits that he was different once. He suffered a loss and now he prefers isolation rather than a return of pain. Vastra and Jenny are his friends but don't approve of his isolation. To test Clara, Vastra tells her to give them a message explaining why the Doctor should help her... in one word.
The Doctor is in the TARDIS reading when his phone rings. It's Vastra, who tells him that she gave Clara the one-word test. The Doctor asks what Clara says and Vastra passes on the word: "Pond."
At the Institute, the Voice tells Simeon that it has detected danger: an intelligence beyond what is possible in the time and place. Before Simeon can question it further, the servant, Bob Chilcott, comes in and says that they have a most insistent visitor: Sherlock Holmes. The Doctor comes in, wearing the traditional costume, but Simeon is well aware that Holmes is a fictional character and orders him out.
Undeterred, the Doctor runs over and hits the globe with his cane. Simeon grabs him and the Voice speaks, saying that it is the Intelligence and knows the Doctor isn't of Earth. The Doctor examines the globe and uses his sonic screwdriver to seal the door when Simeon rings for help. The Time Lord concludes that the Intelligence needs human DNA in ice form to create a body for itself and tosses open a folder that Simeon has been reading. It opens to a news clipping about the governess' death. Simeon's men break in but when Simeon turns back to the Doctor, he discovers that the intruder has slipped out through the window.
The Doctor goes to Latimer's estate and scans the pond. As he works out that the sentient ice conducted a full-body scan, Strax arrives with orders from Vastra to help with the investigation. The Doctor insists that he isn't investigating anything and sends Strax away. Upstairs, Clara looks out the window and waves to the Doctor, who hesitantly waves back. She gestures him up and he goes inside despite himself... as the ice in the pond cracks open.
Vastra, Jenny, and Strax watch the Latimer manor and see Simeon pull up outside.
Clara assures Francesca that she won't have a nightmare and tells her and her brother about how the Doctor is coming to help them and lives on a cloud in the sky and keeps children from having bad dreams. The door opens and Clara assumes that it's the Doctor. However, it's an icy figure of a woman, screaming that the children have been naughty. Clara shields Digby and Francesca behind her, ushering them out of the room and locking the door behind them. As the Ice Governess tries to break the door down, Clara tells Francesca to imagine the monster melting but the girl can't.
As the Ice Governess breaks in, Digby asks where the Doctor is. Suddenly a puppet in a nearby toy theater speaks up and then aims the sonic screwdriver at the creature. It explodes into a million pieces and the Doctor stands up, smiling.
Outside, Simeon activates an antenna-like device which directs a blast of snow at the manor.
The Doctor confirms that the Ice Governess is destroyed and warns Clara that it's not the kind of thing he does anymore. He catches a glimpse of himself in a mirror and realizes that he's put his bowtie back on out of habit, while ice forms on the windows. As the temperature drops, the Ice Governess reforms in the carpet and the Doctor tells the others to run. When they go downstairs, they run into Latimer who demands to know what's going on. The Doctor makes up a story that he's Clara's boyfriend and they've been kissing. Before Latimer can pursue the matter, Alice runs in and tells them that snowmen are growing in the garden. The doorbell rings and Alice opens it to find Vastra and Jenny, while Strax storms in. Alice faints and Vastra informs them that the snow is localized and artificial.
While the Doctor tries to come up with a plan, the Ice Governess starts coming down the stairs after them. Jenny contains it with a forcefield grenade while Strax finds a defensible locale. As they run for the study, Vastra comments that the Doctor must have missed the adventures. Rather than argue, he just smiles and tells her to shut up. They join the others and Strax concludes that the snowmen are in a defensive formation. Vastra believes that there's something in the house that they want and the Doctor deduces that they want the Ice Governess because she's a perfect encapsulation of human DNA in ice form. The snow needs to evolve and the Ice Governess is the blueprint for its army of ice.
The doorbell rings and the Doctor tells the others to stay put while he answers it. Clara insists on going with him and he points out she doesn't listen to him. She figures he likes her and kisses him. The Doctor, embarrassed, blushes and runs to the door. Simeon and his snowman army are there and he gives the Doctor five minutes to release the Ice Governess before walking away.
The Doctor tells Clara that they need to get the Ice Governess out of the house without letting Simeon obtain it. He gives her an umbrella and then expands the forcefield so that he and Clara are inside of it, and then run past it upstairs and to the roof. When Clara tries to get out of the window, her bustle catches and the Doctor pulls her out and tells her that she has to take her clothes off. He hastily apologizes and she says that she understands, and then asks what his plan is. The Doctor tosses her the umbrella and tells her to figure out what his plan is. As the Ice Governess turns into snow and comes out through the window, Clara realizes that since they're neither hiding nor escaping, and then reaches up with the umbrella and snags the invisible TARDIS ladder. The Doctor climbs up and then pulls Clara after him.
The Doctor and Clara reach the stairs and the Ice Governess comes after them, just as the Doctor planned so she's stay away from the snow. As they climb, the Doctor asks Clara if she's a governess or a barmaid, but Clara avoids the question. They get to the cloud level and the Doctor condenses the super-dense water vapor even further, trapping the Icy Governess inside. He then leads Clara into the TARDIS and explains that it travels through time and space. As she looks around, Clara wonders if it has a kitchen, explaining that she likes to make soufflés. The Doctor, remembering Oswin, her love of soufflés, and her similar interest in soufflés, asks Clara why she followed him. Clara points out that he's tall enough to have reached the ladder without the umbrella and that he gave it to her deliberately so she could follow him. She asks why she brought him along and the Doctor says that never knows why, only who. He gives her a TARDIS key and says that it's him, giving in.
As the Doctor tells Clara that today is the day that everything begins, the Ice Governess enters the TARDIS, grabs Clara, and drags her outside. The Doctor goes after them but can only watch as the Ice Governess plummets off the edge of cloud, taking Clara with it.
In the manor, Jenny and Vastra see Clara hit the ground. They use a scanner and confirm that she's dead, and the Doctor materializes the TARDIS around Clara's body and the Ice Governor's shards. He then takes the TARDIS into the manor and Strax tries to revive Clara. Meanwhile, Vastra goes to see the Doctor, who is examining the ice shards. He tersely tells the Silurian that it can't form inside the TARDIS and insists that Clara will be fine. Vastra warns that Strax's equipment can't keep Clara alive forever and the Doctor blames himself for what happened... but then says there's no point because Clara will live.
The Doctor goes out and hands a box with the ice shards to Jenny. Clara wakes up and the Doctor assures her that he knows she will live and places the TARDIS key back in her hand. When she asks if he'll save the world, the Doctor asks Clara if she'll come with him if he does. Clara agrees and the Doctor wishes her a merry Christmas. He then turns away, adjusts his bowtie, takes the box back from Jenny, and goes to the front door. The Doctor holds up the box and asks if it's what they want, and then slams the door shut and goes to the TARDIS. Vastra goes in with him and as the Doctor prepares for dematerialization, she asks him if he's saving the world to keep his bargain. The Doctor insists that the universe owes him and sets the TARDIS in flight.
Simeon returns to the Institute and finds the Doctor and Vastra waiting for him. The Doctor points out that the Intelligence hasn't said anything while Simeon has been gone, and then holds up the box and explains that it has markings on it representing the London Underground in 1967. As the Intelligence explains that it needs to take human form, the Doctor uses his sonic screwdriver to scramble the entity's voice, stripping away its disguise. The Intelligence speaks in the voice of Simeon as a child, and Simeon begins to collapse. The Doctor explains that the Intelligence is just a mirror of Simeon's thoughts and fears, and he's using it to create an army of ice people: a Victorian concept.
Simeon grabs the box, insisting that he's sure that's what he wants. However, what's inside the box is a memory worm. It bites him and he collapses, while the Doctor explains that the parasitical snow is harmless without a host. The voice of the Intelligence falters... and then gains new power, saying that it's not that easy. Outside, the telepathic snow starts falling in force and the snowmen begin growing.
The Doctor turns to the globe and asks how it can exist without Simeon, and the Intelligence says that the dream has outlived the dreamer. It enters Simeon's mind, using the space cleared by the memory worm, and bats Vastra aside. Simeon grabs the Doctor and freezes him with a touch
Clara's lifesigns begin to fade and Latimer comes to her side. She takes his hand and tells him that his children are afraid and need their father to hold them. When he admits that it isn't his area, Clara tells him that he has to now as she sheds a single tear.
The snow across London suddenly turns to rain and the possessed Simeon collapses. The Doctor watches as the old man's body dies, and realizes that the snow is mirroring something so strong at the house that it's drowning everything else. He goes to the window and samples the rain, and realizes that it's salty: tears, not rain.
The Doctor and Vastra return to the manor and find Latimer and his children crying. Strax tells the Doctor that there was nothing he could do and Clara only has a few moments left. The Doctor goes to Clara and tells her that the two of them saved the world together. He assures her that he won't go back into isolation and Clara tells him that he's a clever boy with her dying breath. The clock chimes midnight and they realize that it's Christmas Day.
Later, the Doctor, Vastra, and Jenny attend Clara's funeral. As they watch from a distance, they wonder the Intelligence poses any further threat and the Doctor finds the idea of a "Great Intelligence" and snowmen vaguely familiar. Once the Latimers leave, the Doctor goes to Clara's grave and discovers that her full name was Clara Oswin Oswald. He remembers Oswin, the girl from the Daleks' asylum planet, and realizes that her and Clara are one and the same, and both said the same thing about him being a clever boy as they lie dying. The Doctor figures that something impossible is going on and tells them to stay there while he leaves in the TARDIS to find Clara. As he departs, Vastra wonders if the universe makes bargains after all.
In the present, a woman and her friend visit the cemetery. One of them, Clara, looks at the tombstone of Clara Oswald and says that she doesn't believe in ghosts.
The Doctor goes to the TARDIS and mutters to himself, saying that Clara should watch him run.