Above a cloud-covered world, a passenger spaceship falls out of orbit and plummets toward the surface. The captain detects a distress signal coming from the honeymoon suite, and tries to determine who it is. Amy and Rory enter the bridge, and Amy informs the captain that she’s sent a signal to a friend who can help. A small shuttle-sized capsule approaches the spaceship and they realize that it’s the Doctor.
On the planet below, in the town of Sardicktown, Kazran Sardick surveys the town from his great tower, and then turns to a family of petitioners, the Pettigrews, who are asking him to release their wife’s sister, Abigail, for one day. She’s encased in a cryogenics chamber, and Kazran mocks their concerns, noting that she’s frozen as the security against their loan. The President calls Kazran and says they have to let the spaceship land. Kazran refuses to entertain the idea, but is interrupted when they hear the TARDIS materialize on the roof. As Kazran informs the President they have too many people already and can’t let four thousand passengers and crew onto the planet, the Doctor comes down the chimney. He introduces himself and admits he couldn’t resist a chimney on Christmas Eve.
The Doctor notices a control console and deduces that it’s tied to the power spire in Kazran’s dome, and it controls the clouds. He comments on Abigail, but Kazran dismisses her as no one important. He then returns to the console and tries to use it to alter the clouds and bring the spaceship down safely. However, Kazran explains that the controls only respond to him. He’s tamed the sky, and is shocked that the Doctor doesn’t know him. The Doctor asks him for help, but Kazran tells him to make an appointment. In response, the Doctor says that he won’t let him stop saving the four thousand and three people aboard the liner, and warns that whatever happens that night, Kazran has no one to blame but himself. Kazran, unimpressed, has his guards escort out the Doctor and the family. As the boy is taken out, he hits Kazran in the head with a coal. Kazran lunges forward to slap him, but hesitates and then tells his guards to get him and his family out.
The Doctor stays behind and asks Kazran why he didn’t hit the boy. Kazran tells him to get out, but the Doctor looks around the room and realizes that there is a portrait of Kazran’s father Elliot, which Kazran refuses to sit near. There’s a Christmas tree in the painting, but none in the house on Christmas Eve. The Doctor says that Kazran isn’t like his father, which is good, because he didn’t hit the boy. He wishes Kazran Merry Christmas, but Kazran declares that he hates Christmas. The Doctor says that Kazran is halfway out of the dark and then walks out.
Aboard the spaceship, Amy gets a communication from the Doctor. He informs her that he can use Kazran’s machine to clear their flight path so they can land safely. The family father, Eric, interrupts the Doctor that no one has ever stood up to Kazran before. He warns the Doctor to get inside before the fish come out, and says that Kazran lets a few fish through the cloud layer when he’s in a bad mood. The Doctor has no idea what he’s talking about. Amy relays the captain’s information that they have less than an hour before they crash, but the Doctor is intrigued by a school of fish circling a nearby streetlight, swimming in the fog. He realizes that the fish are in the cloud bank, and warns them to be careful. Christmas music starts playing on the streetlamp loudspeakers, and the Doctor tries to work out a way to land the ship. When he hears the Christmas carol playing, he gets an idea.
Kazran is sleeping alone in his apartment and dreaming of himself as a 12-year-old working on a secret project. He wakes up and realizes there’s a recording playing on the wall. In it, his father Elliot storms in and warns him against making a film about the fish. He wants to use singing to attract the fish, but Elliot says that it’s nonsense. Kazran watches the recording, which shows Elliot slapping his childhood self. The Doctor emerges from the shadows and tries to reassure Kazran, who demands an explanation. The Doctor explains that he found the recording on an old drive. When Kazran tries to summon his servants, the Doctor informs him that they all quit after winning the lottery at the same time. Kazran looks at the recording, which shows his younger self crying as his Elliot storms out. The Doctor asks if he ever got to see a fish, and Kazran admits that he learned life’s most valuable lesson: nobody comes. He yells at the Doctor to get out. As the Doctor enters the TARDIS, he says he’ll leave for now but will be back... way, way back.
As Kazran watches the recording of past events, the Doctor appears to his younger self and introduces himself as his new babysitter. He leans into the recording and addresses Kazran, and says that as his past changes, his memories will change but he’ll soon get the hang of it. As he looks on in horror, Kazran remembers the Doctor meeting him when Kazran was a child.
In the past, the Doctor bounces around Young Kazran’s room and suggests they talk about girls. Young Kazran explains that Elliot invented a machine to tame the clouds, and the fish can only come down when he lets them. The Doctor says that it’s a boring alternative, and offers to show Young Kazran a fish. He rigs his sonic screwdriver as a lure to bring a fish in through the open window while they hide in the cupboard. Young Kazran admits that he’s interested in fish because there was an attack at his school, and he wasn’t there to see it because he was out sick. Now everyone has a story about that day except him. As they talk, the Doctor records their conversation for the benefit of Kazran in the future.
When they get a bite, the Doctor steps out of the cupboard and finds a single fish poking at the sonic screwdriver. Suddenly a shark swims in the window and eats the fish and the sonic screwdriver, and the Doctor tells Young Kazran to stay in the cupboard. He runs inside and tells Young Kazran that after studying the fish, he realizes how he can land the ship in the future. He finally gets around to telling the boy that there’s a shark in his bedroom. The shark slams through the door...
And in the future, Kazran watches in horror as the recording stops.
The shark tries to get through the hole in the door, and the Doctor realizes it’s swallowed his sonic screwdriver. He reaches in, turns it on, and stuns it. Young Kazran realizes that it’s dying, and the Doctor admits that it can’t survive for long outside of the cloud belt. The boy doesn’t want it to die, despite the fact it was trying to eat him. The Doctor has restarted the recording device, and in the future, Kazran watches and cries. They need a fully-functional life support unit like an ice box, and Young Kazran takes the Doctor to the basement. The surplus population is stored in ice boxes, and the door is locked. Kazran knows the combination, and yells it out, but his younger self admits he doesn’t get to know it until he’s older. The Doctor pokes his head out from the TARDIS, hears Kazran yell it, thanks him, and then returns to the past. They open the door and enter the chamber, and Young Kazran explains that the house is built on a fog bank, which Elliot uses to freeze the people.
Inside the chamber, the Doctor finds the ice box holding Abigail. Young Kazran plays a recording she made saying she’s thankful for Elliot’s kindness, believing he’s a philanthropist. Young Kazran explains that Elliot loans money and takes a family member as security. As they talk, the Doctor’s half of the sonic screwdriver starts repairing itself, and the Doctor realizes that the shark with the other half in its stomach is homing in on it. The shark swims in, knocking the Doctor aside. Young Kazran runs between the ice boxes and hides, and the shark circles nearby.
As Young Kazran hides, he hears a woman singing. Emerging, he discovers that Abigail’s ice box is open. She’s singing to the shark, calming it. The Doctor realizes that the singing triggers a resonance in the ice crystals, and that’s how Elliot controls the clouds.
In the future, Kazran smiles at the memory, and turns to look at his father’s portrait. It has transformed into a portrait of Abigail.
In the past, the Doctor brings the TARDIS down to the cellar and gets the shark into Abigail’s ice box. He then takes them all up into the cloud banks and releases the shark. As he does so, Young Kazran takes a picture of Abigail.
In the future, Kazran takes out the photo of Abigail.
The shark swims out into the cloud bank, while the Doctor notices that there’s an 8 on the counter of her ice box. Abigail asks if the Doctor is one of her doctors. The Doctor takes them back and puts Abigail into another ice box. As they close the door, Young Kazran says that the Doctor will come back every Christmas Eve. One year later, or a second from Abigail’s perspective, the Doctor and Young Kazran comes back and release her for Christmas Eve. As they go, they’re unaware that the counter on her ice box has decreased to 7.
On the roof, the Doctor prepares a sled and uses his sonic screwdriver to summon the other half. The shark swims down and lifts them up into the sky. They travel across the city and then return, putting Abigail in her ice box and promising her an even better Christmas Eve in a year.
In the future, Kazran looks over the photos representing his new memories as the Doctor took Young Kazran and Abigail on a trip through time and space. Each year Kazran grows a year older, and the counter on Abigail’s ice box counts down another year. Abigail finally makes a request to stay on the planet and visit her family, the Pettigrews. She watches from outside as her family celebrates, and they notice her crying. Adult Kazran goes over to talk to her, and wonders why she isn’t happy when they are. She takes his hand and explains that she can’t be with them, but the Doctor goes inside and invites her and Adult Kazran in. Abigail explains to her sister, Isabella, what the Doctor has done for her every Christmas Eve. When Isabella warns that Kazran will be no different than Elliot, Abigail insists that he’s different. Isabella asks her to stay for Christmas dinner, but Abigail says that she can’t. Isabella says they’ll have dinner that night instead. Adult Kazran offers a toast while holding hands with Abigail.
Back in the basement, Abigail says it was the best Christmas Eve ever, and the Doctor says it will be until the next one. She asks the Doctor to leave, and Adult Kazran asks what to do if Abigail kisses him. With some prodding from the Doctor, they kiss.
In the future, Kazran finds more photos, including one of a Christmas Eve in California in 1952. Adult Kazran finds Abigail alone by the pool. He asks her what’s wrong, and Abigail starts to tell him the truth. A minute later, the Doctor runs out, saying he just got accidentally engaged to Marilyn Monroe, but finds them kissing. When he can’t get their attention, he goes to get married. Adult Kazran asks what they’re going to do, and Abigail admits that there’s nothing they can do. Back on the planet, Adult Kazran returns Abigail to her ice box and says good night to her. As the Doctor prepares to leave, Adult Kazran says that it’s time to put an end to their Christmas Eve tradition, and he has work with his father to focus on. As Adult Kazran starts to walk away, the Doctor gives him his sonic screwdriver and says to activate it if he ever needs him. Adult Kazran says he won’t need him again, and the Doctor asks what he’s hiding. He gets no response and asks about Abigail, and Adult Kazran says he knows where to find her. As he walks away, the counter on Abigail’s ice box counts down to 0.
In the future, Kazran turns back to the portrait, which now shows his father Elliot again.
The next year, Elliot shows Adult Kazran the completed mechanism for controlling the cloud banks using sound waves. He can also control the fish, and plans to control the people, the cattle, by controlling their predators. When Elliot says that the planet is now theirs, Adult Kazran walks away and goes to get the sonic screwdriver. As he examines it, he realizes the Doctor is outside at the window. Adult Kazran closes the drapes on him and puts the sonic screwdriver back in the drawer with the relics of their trips through time.
In the present, Kazran takes the sonic screwdriver out of the dusty drawer and contemplates it. The President calls and Kazran says that the crashing spaceship has nothing to do with him. He tells the President that like an old friend took a long time to explain, life isn’t fair. As he hangs up, Kazran considers the photographs of his Christmas Eve trips. Suddenly, a hologram transmission of Amy appears. She says that it isn’t over, and she’s the Ghost of Christmas Present. After a brief argument with Rory, she disappears, but Christmas music fills the room. Kazran goes to the basement and discovers that people are singing. Amy appears behind him and explains that they’re hologram projections like her, the people on the ship who are going to die because of him. They’re singing for their lives, but Kazran tells Amy to tell the Doctor that people can’t change.
He stalks off through the holograms and goes to Abigail’s ice box. He informs Amy that if it hadn’t been for the Doctor, he never would have known Abigail. When she asks why he doesn’t release her, Kazran explains that Abigail was on the brink of death when she went into the ice box. She’s used up her time on all those Christmas Eves with him, and would only survive a single day if he released her. Kazran asks her how he chooses what day she will survive. Amy apologizes and then broadens the hologram beam, showing him what’s happening on the ship. He hears the singing and Rory explains that the Doctor has them singing to resonate the ice crystals and stabilize the ship. However, it’s not working. The passengers are still singing because they haven’t told them that the plan won’t work.
When Kazran refuses to save them, Amy calls the Doctor and shuts down the projector. The Doctor is waiting in the basement, and apologizes for not realizing. Kazran says that thanks to the Doctor, his heart is broken, and insists that it’s worse than having no heart at all. The Doctor says that it’s time for Kazran to see his future, but Kazran says that he knows his future is to die alone, and he doesn’t care. In response, the Doctor says he doesn’t believe it and explains that he’s already showing Kazran his future. Kazran turns and discovers that he’s in the past, and his younger self has heard the entire thing. Kazran raises his hand to hit Young Kazran, and then remembers everything that he’s done and everything that’s happened. He breaks down, sobbing, and apologizes to his young self. The Doctor warns that they don’t have much time.
The ship has five minutes remaining. The Doctor calls and tells them to lock onto his signal. On the planet, Kazran discovers that the controls won’t respond to him, and the Doctor realizes that he’s changed history and altered Kazran’s history so that Elliot never programmed the machine to recognize the man he is now. Kazran offers him the half of the sonic screwdriver, and the Doctor gets an idea. He sets up a resonance between the two halves, and uses the signal to send something else. However, there’s only one thing they can transmit that they know will calm the shark, and the sky: Abigail’s singing voice.
Kazran releases Abigail, who is surprised to see he’s still old. He warns her that she’ll die if he releases her to save the liner, but she says it’s time for Christmas Day.
As the ship goes down for the final time, they hear Abigail’s voice reverberating through the clouds. On the voice below, the Doctor explains that when the two halves of the sonic screwdriver resonate properly, the clouds will unlock.
The ship stabilizes and they realize they can bring it down for a landing. Rory and Amy kiss.
On the planet, it snows for the first time and the people come out into the streets to celebrate. As Abigail and Kazran celebrate together, the Doctor quietly slips away to take Young Kazran back to his own time.
Later, Amy and Rory find the Doctor making snowmen. As they get into the TARDIS, Amy asks if the Doctor is okay. She asks if it will be Abigail and Kazran’s last day, and the Doctor admits it is, but everything has to end so everything can get started. Rory informs the Doctor that Marilyn is calling, and the Doctor assures Amy that Abigail and Kazran are off on a little trip for Christmas. The Doctor looks up to the sky and then leaves.
Somewhere up in the sky, Abigail and Kazran go on one last sleigh ride with their friend, the shark.