The Doctor hauls Clara into the carriage. She shouts to be let out. The Doctor examines the snow and asks Strax, who is driving the carriage, what he would look for after finding something that he's never seen before. Strax replies" a grenade". The Doctor tells Strax to fetch a memory worm to erase Clara's memory. Strax arrives without the worm; he forgot to wear the protective gauntlets. When Strax is sent under the carriage to get the worm he forgets the gauntlets again and thinks he's been run over. Amused, Clara gives the gauntlets to the Doctor, who replaces the memory worm into a jar.
Clara asks about the snowman as one suddenly erupts out of the snow. The Doctor asks Clara if she was thinking about it as more snowmen rise up. The Doctor says that the snow is sentient and reflects the thoughts of people near it. The Doctor tells Clara to imagine the snowmen melting. The Doctor cannot erase her memory now because she would forget how to defend herself. He orders her to forget him. He tells Strax to take her home but she sneaks out of the carriage and follows the Doctor.
He enters a park and pulls a ladder out of thin air above him. She lets him start his ascent and then pulls the ladder down and follows. She climbs enormously high to a platform. Leaning over, she waves to people below but they don't seem to see her. Clara continues up until she reaches the clouds where the TARDIS waits. She knocks on the door and hides round the side of the TARDIS as the Doctor leans out. While the Doctor looks for her, she runs back down the stairs. The Doctor finds a cloth that she dropped and watches her go.
The following day is Christmas Eve. Clara returns to her job as governess to the children of Captain Latimer, Franny and Digby. She uses the alias Miss Montague. Captain Latimer is having trouble connecting with his children and hopes Clara can communicate with them. He says that Franny seems troubled. When she goes to meet them they plead with her to use her secret voice (her real Cockney accent). The children tell her they prefer her to their last governess who drowned in their pond the previous year.
Dr Simeon arrives at the Latimer household to examine the frozen pond. Clara is perturbed but thinks the Doctor may be able to help her.
Clara returns to the park but can't reach the hidden ladder and shouts up to the Doctor. Jenny watches and then takes her to see Madame Vastra. Clara is bothered by Vastra's appearance but overcomes her qualms. Vastra tells her that she will pass a message on from Clara to the Doctor who is retired now after suffering a great loss. The problem is that the message may only be one word long. When the Doctor finds out that the word Clara gave was 'pond' he is intrigued enough to investigate. The Doctor goes to Dr Simeon's institute in the guise of Sherlock Holmes. He examines the glass sphere and observes a newspaper article about a governess who drowned in a pond. The Doctor deduces that Clara said 'pond' to warn him. While Simeon's servants break through the doors which the Doctor has locked the Doctor sneaks out.
He visits the Latimer home to examine the pond where the woman drowned. As he does so, he notices Clara waving through a window and decides to talk to her. Clara is telling Franny and Digby a bed-time story about the Doctor and how he stops children from having bad dreams. Franny reminds her that she has been having nightmares about the governess who drowned. When the door opens Clara expects the Doctor to enter but it is a facsimile of the drowned woman made from ice. The Ice Governess attacks the children but Clara locks them in a playroom. The Ice Governess breaks but the Doctor shatters her with his sonic screwdriver.
The Ice Governess begins to reconstitute herself so the Doctor, Clara and the children escape downstairs and bump into Captain Latimer. The Doctor explains he is there because he is Clara's gentleman friend and they had been upstairs kissing. Amid the confusion Dr Simeon arrives outside the house, using a device that causes snow to fall on the frozen pond.
Vastra, Jenny and Strax enter the house and set up a force field to isolate the Ice Governess. Strax decides that Captain Latimer's office is the best place to defend so they hide out in there. The Doctor explains that Simeon's plan is to use the Ice Governess to an army of ice out of the snowmen. Simeon will use them to conquer the Earth. The Doctor tells the others to stay in Latimer's office but Clara follows him and kisses him in the corridor. The Doctor encounters Simeon at the front door and is told that he has five minutes to hand over the Ice Governess. The Doctor grabs an umbrella and then uses the sonic screwdriver to turn off the force field. He runs upstairs to the roof followed by (and led by) Clara.
They are pursued by the Ice Governess. Clara uses the umbrella to pull down the ladder up to the spiral staircase and they run up to the TARDIS. The Doctor is trying to get the Ice Governess away from the power source that allows her to reconstitute herself. Ultimately he plans to destroy the creature and quarantine the pieces in an impenetrable container.
The Doctor leads Clara inside of the TARDIS; instead of saying it is bigger on the inside she says, "It's smaller on the outside." She also asks if there is a kitchen because she enjoys making soufflés. He gives Clara a key to the TARDIS but as she takes it Clara is grabbed by the Ice Governess and both of them tumble to the earth below. The Ice Governess is shattered and Clara mortally injured.
The Doctor takes Clara into Latimer's study. Strax tries to revive her but she is too badly injured to save. The Doctor asks Clara if she would like to travel with him and she says yes.
The Doctor confronts Simeon and his snowmen. He has a box which contains the Ice Governess. He tells Simeon to meet him at the institute. Vastra asks if the Doctor is making a bargain with the universe: the Doctor says that the Universe owes him, hoping it will allow Clara to survive.
Back at Simeon's office, the Doctor and Vastra force a showdown with Simeon. The Doctor reveals that the giant snow globe contains a version of Simeon's essence: Simeon recalls the snow speaking to him when he was a child; the Doctor thinks that he put all of his darkest thoughts into the snow, making it intelligent. Simeon is amazed to find that he made the snow globe sentient but this does not deter him. He takes the box from the Doctor and opens it, only to discover the memory worm. It erases the memories of his adult life.
The snow globe still intends to continue with the plan and now invades Simeon. Simeon attacks the Doctor but then is halted in agony when the snow in the globe turns to rain. Simeon dies and the entity within is defeated. The Doctor says that the snow back at the Latimer house was reflecting the emotional pain of the Latimers as Clara approaches death. The Doctor races to be at Clara's side for her last moments. Before she dies Clara says, "Run you clever boy. And remember."
With her death overwhelming him, the Doctor seems to have returned to his previous withdrawn and angry state. He attends Clara's funeral where Vastra shows him a card for Simeon's Institute which shows that the entity in the snow globe was the Great Intelligence. The name sounds familiar but the Doctor can't quite remember where from.
Clara's grave gives her full name as Clara Oswin Oswald. The Doctor remembers that from the Dalek Asylum, Oswin Oswald who enjoyed making soufflés. The Doctor is puzzled because the same woman has apparently died twice in different times. Now revitalized, he realises that there is a chance of meeting her again: he has a puzzle to solve and relishes the challenge.
In the 21st Century, the graveyard where Clara's body lies is overgrown. Another Clara passes near the grave but her friend asks her to leave because it is spooky. Clara says that she doesn't believe in ghosts.
In the TARDIS the Doctor puts a picture of Clara on the TARDIS screen and says, "Watch me run!"