Walter Simeon

In England 1842, a young boy is building a snowman but refuses to play with the other children. The snowman starts speaking to the boy, repeating his assertions that the other children are ‘silly’ and that he ‘doesn't need anybody’. Fifty years later, the boy has grown up to be Dr Simeon, proprietor of The Great Intelligence Institute. He hires men to collect samples of snow, which he places in a large snow-filled globe in his laboratory. He then later feeds the men to a group of animated snowmen.

Meanwhile, The Doctor, still despondent after losing his former travelling companions Amy Pond and Rory Williams, has parked his TARDIS above London among the clouds, descending to the surface via a long circular staircase. He wants nothing to do with what is happening on Earth despite three of his allies, the Silurian Madame Vastra, her human companion Jenny Flint, and the Sontarian Strax, scouting around London looking for any oddities even when Madame Vastra and Jenny encounter Dr Simeon and follow him, curious as to his interest in the snow.

Elsewhere, Clara Oswald, a barmaid, investigates a disturbance outside an inn called The Rose & Crown, where she works. She is surprised to see a snowman standing there that wasn't there before. Then The Doctor walks by. After accusing him of creating the snowman The Doctor realises that it is made of snow with some type of memory. As more snowmen start to appear and become animated with intent to harm the pair, The Doctor tells Clara that her thoughts are creating the snowmen, and so as to get rid of them she needs to think of them melting; after she concentrates, the snowmen turn into water.

Clara Oswald

The Doctor then attempts to leave discreetly, but as she sees him drive away in a horse-drawn carriage, Clara runs after him as she is curious about his actions. On discovering that she has followed him The Doctor instructs Strax, waiting nearby, to bring a memory worm, with the intention to use the creature's touch to wipe away the last hour of Clara's memory, in particularly her knowledge of meeting him. After Strax accidentally wipes his own memory, Clara retorts that if he does so, she will forget how to deal with the snowmen should they appear again. The Doctor lets her go, and thinking he has evaded her, takes the staircase back to the TARDIS. Clara however, follows him and finds the TARDIS. When she knocks on the door she panics and hides at the side of the TARDIS just as The Doctor pops his head out. As The Doctor walks around the TARDIS, Clara sneaks down the spiral staircase. The Doctor hears her footsteps and then sees her running down but decides not to follow as he is still determined to stay out of what is happening on Earth.

The following morning Clara returns to her other job as the governess for the children of Captain Latimer, replacing the former governess who had drowned and then frozen in his mansion's pond a year before. She finds out that Captain Latimer's daughter has been having nightmares about the old governess returning from her icy grave in the pond and killing them all. Concerned about this Clara attempts to contact The Doctor but instead attracts the attention of Jenny, who takes her to see Madame Vastra who, after questioning her, tells Clara she gets only one word to impress The Doctor with if she wants his help. After some thought Clara tells him ‘Pond’ – referring to the frozen pond and not realising that it is also the surname of The Doctor’s former travelling companion. On hearing Clara’s word The Doctor is spurred into action.

The Doctor

The Doctor visits Dr Simeon's laboratory, and finds that the giant snow-filled globe contains The Great Intelligence, the entity that has been speaking to Dr Simeon since he was a child. The Doctor learns that The Great Intelligence has been controlling the snowmen and has taken great interest in Captain Latimer's pond. He goes there to investigate just as a humanoid ice creature, in the form of the former governess, rises out of the pond and enters the mansion. The ice creature attacks Clara and the children, but The Doctor appears just in time to destroy it. Madame Vastra, Jenny and Strax then arrive just as Dr Simeon and his snowmen cover the mansion in ice. The Doctor and Clara are then horrified when a reanimated ice creature resumes its attack.

After assuring that that his allies will protect Captain Latimer and his children, The Doctor flees with Clara to the roof of the mansion followed by the ice creature. The Doctor and Clara then ascend the circular staircase to the TARDIS which The Doctor has moved to above Captain Latimer's mansion. However, they are followed by the ice creature and so The Doctor creates a barrier at the top to halt the ice creature's advance.

The Doctor then introduces Clara to the TARDIS; while as impressed with its transdimensionality as others, her replies are curious oddities to The Doctor, such as noting how it's smaller on the outside, or asking if it has a kitchen for making soufflés. The Doctor explains that he has been reluctant to gain a companion, but now considers her to be one, giving her a TARDIS key. However, just as Clara takes it, the ice creature, having broken through the barrier, grabs her from behind and pulls her out of the TARDIS and they fall off the cloud, causing Clara to fall to her death in the Captain Latimer's courtyard, before The Doctor can do anything to save her.

The Snowmen

The Doctor helps to recover the barely-alive Clara from the army of snowmen with his TARDIS and returns to the mansion, where Strax uses alien technology to revive her. However, Clara's injuries are fatal and even the resurrection technology isn't going to be enough to save her. Leaving Strax to do the best he can do The Doctor collects some ice fragments from the ice creature, assuring they remain dormant but finding they contain ice-based DNA, the material that The Great Intelligence is looking for, and apparently places them in a souvenir London Underground biscuit tin. He and Madame Vastra travel to Dr Simeon's lab, where The Doctor reveals The Great Intelligence's plan to replace humanity with ice creatures, and holds up the tin, stating that it contains the ice DNA that is necessary for the plan. Dr Simeon grabs the tin, but opens it to find it contains the memory worm. It bites Dr Simeon; The Doctor (who previously noted that the worm's bite would erase decades of memory) states that The Great Intelligence, which has been existing as a mirror of Dr Simeon's thoughts, will vanish with the erasure of Dr Simeon's memories. Instead, The Great Intelligence reveals that it has existed long enough that it can now control Dr Simeon's body, which it uses to attack The Doctor.

Just as Dr Simeon is about to kill The Doctor, the influence of The Great Intelligence wanes greatly, and Dr Simeon, now dead, falls to the floor. Outside, a salt-water rain has started, and The Doctor realises that some other, more powerful psychic ability has taken control of the snow from The Great Intelligence. The Doctor deduces that it must be Clara, crying as she nears her death, and with Madame Vastra, he quickly returns to Captain Latimer's mansion in the TARDIS. Strax informs The Doctor that Clara only has moments left. Realising that there is nothing he can do The Doctor tries to cheer Clara up, returning the TARDIS key to her just before she dies.

At her funeral, The Doctor reads Clara's full name, Clara Oswin Oswald, on her tombstone and realises she is the same woman he met, turned into a Dalek, in "Asylum of the Daleks"; he never had seen Clara's face then, but now recognises the similarities in voice and their fascination for soufflés. He gleefully announces that the same person dying twice is an impossibility he must investigate, and says his goodbyes to his allies before departing in the TARDIS.

In contemporary times, a young woman, identical in appearance to Clara Oswald and Oswin Oswald, walks, with a friend, through the graveyard where her namesake is buried. Back in the TARDIS, The Doctor is heard yelling ‘Clara! Oswin! Oswald!’ as he pulls up a picture of Clara from 1892 on to the TARDIS video screen. As The Doctor dashes around the TARDIS console he echoes Clara's dying words: ‘watch me run!’.