Doctor: Matt Smith (Eleventh Doctor)
Jenna-Louise Coleman (Clara Oswin Oswald)
Richard E. Grant – Dr Simeon
Dan Starkey – Strax
Catrin Stewart – Jenny Flint
Neve McIntosh – Madame Vastra
Tom Ward – Captain Latimer
Liz White – Alice
Joseph Darcey-Alden – Digby
Ellie Darcey-Alden – Francesca
Ian McKellen – The Great Intelligence
Juliet Cadzow – Voice of the Ice Governess
Jim Conway – Uncle Josh
Cameron Strefford – Young Walter
Annabelle Dowler – Walter's Mother
Ben Addis – Bob Chilcott
Daniel Hyde – Lead Worker
Sophie Miller-Sheen – Girl
Directed by Saul Metzstein
Written by Steven Moffat
Produced by Marcus Wilson
Incidental music composer Murray Gold
Length 60 minutes
Originally broadcast 25 December 2012
In 1842 England, a young boy builds 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. Fifty years later, the boy has grown up to be Dr. Walter 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 before feeding the men to a group of animated snowmen. The Doctor, still despondent after losing his former companions Amy Pond and Rory Williams, has parked his TARDIS above Victorian London among the clouds. He uses his allies Vastra, Jenny, and Strax to keep people away from him. They also fill their time investigating mysteries throughout the city.
Elsewhere, barmaid Clara Oswin Oswald investigates a disturbance outside the tavern she works at and finds the Doctor walking by. He attempts to leave discreetly, but Clara follows him to a coach. Not wishing to become involved in matters, the Doctor instructs Strax to bring him a memory worm that will erase the last hour of Clara's memories with just a touch. Before they can do so, they are surrounded by snowmen created from snow with psychic properties who attack the group. The Doctor realises that Clara's thoughts are creating the snowmen and ends the threat by instructing her to think of them melting. Clara cautions the Doctor that if he wipes her memory, she will forget how to deal with the snowmen. The Doctor reluctantly allows her to go and ascends a staircase to the sky to return to the TARDIS. Clara follows him and knocks on the door, but she hides and flees down the staircase when the Doctor answers. Clara returns to her other job as governess for the children of Captain Latimer. She learns that Latimer's daughter has been having horrible dreams about their previous governess returning from the dead. Clara realises that the pond that contains the old governess' body is the only thing still frozen around them. She attempts to track down the Doctor but instead attracts the attention of Jenny, who takes her to see Vastra. Vastra tells Clara she gets only one word to impress the Doctor with if she wants his help. Clara chooses the word "Pond", which shocks the Doctor and arouses his interest.
Acting on a tip from Strax, the Doctor visits the Great Intelligence Institute posing as Sherlock Holmes. He confronts Dr. Simeon and find a large glass globe in Simeon's office that contains psychic snow. The Doctor speaks to the Great Intelligence, the entity that has been speaking to Dr. Simeon since he was a boy. He learns that the Great Intelligence has been controlling the snowmen and has taken interest in Latimer's pond. The Doctor visits the pond and deduces that the Great Intelligence is using the old governess' body as a DNA blueprint to form an ice creature that will retain its form and not melt. While Clara is putting the children to bed, the frozen body of the governess breaks into the house. The Doctor fights her off and is joined by Vastra, Jenny and Strax. Dr. Simeon arrives with more snowmen and tells them he wants the governess' ice body. The Doctor flees with Clara to the roof of the mansion and then to the TARDIS hovering overhead. They are pursued by the ice governess, whom the Doctor traps under a layer of frozen ice crystals. Inside the TARDIS the Doctor gives Clara a TARDIS key, but the ice governess arrives and pulls Clara down off the cloud.
The Doctor picks up Clara and takes her back to Latimer's mansion, placing her under medical care of Strax. He collects the ice fragments from the governess and places them in a souvenir London Underground biscuit tin. He and Vastra travel to Simeon's lab, where the Doctor notes the Intelligence's plan to replace humanity with ice creatures and holds up the tin with the necessary DNA. Dr Simeon grabs the tin and opens it only to find the memory worm, which latches on to him. The Doctor states that the Great Intelligence, which has been existing as a mirror of Dr Simeon's thoughts, will vanish with the erasure of Simeon's memories. Instead, the Intelligence reveals that it existed long enough that it can now control Simeon's body, which it uses to attack Vastra and the Doctor. The influence of the Great Intelligence quickly wanes, and Simeon falls dead. Outside, a salt-water rain has started, and the Doctor sees that another psychic ability has taken control of the snow from the Great Intelligence: the Latimer family, crying for Clara. Strax informs the Doctor upon his return to the Latimer mansion that Clara only has moments left, and she passes away as the Doctor returns the TARDIS key to her. At her funeral, the Doctor reads Clara's full name on her tombstone and realises she is the woman he met in "Asylum of the Daleks" who became a Dalek, whom he refers to as "Soufflé Girl". He gleefully announces that a person dying twice is an impossibility and, bidding farewell to his allies for now, the Doctor departs in the TARDIS to investigate and find Clara. The episode concludes in contemporary times, where a young woman resembling Clara walks through the same graveyard, pausing by Clara's tombstone.