Donna Noble is coming down the aisle during her wedding to Lance Bennett on Christmas Eve. However, she disappeared in a glow of golden energy and drifts through the ceiling.
In the TARDIS, the Doctor stares in astonishment as Donna appears in the console room. She's equally astonished and demands that the Doctor return her to her wedding. When she tries to leave, she discovers that the TARDIS is floating in deep space. Realizing she's aboard an alien craft and the Doctor is himself an alien, she still insists on his returning her to the church. As they argue, she notices Rose's jacket nearby and accuses him of having abducted other Earth women. The Doctor is unhappy to be reminded of his companion's recent departure.
Trying to get rid of Donna, the Doctor tries to return he to the church but materializes behind some business offices. The Doctor tries to hail a taxi but soon realizes he has no money so uses his sonic screwdriver to get some from an ATM. He also helps Donna make a call to her mother but her mother Sylvia's cell phone dead and Rose is forced to leave a message.
The Doctor notices three Santas approaching and realizes that they're alien roboforms. He's unaware that Rose has borrowed the money to pay for a cab. However, as she departs she's unaware that a fourth roboform Santa is driving. The Doctor causes money to eject from the ATM, distracting the three attacking roboforms, and then runs to the TARDIS and pursues the taxi.
Donna finally realizes that her abductor is a sinister Santa. She has nowhere to escape until the Doctor pulls up in the TARDIS, sets it on course, and reaches out the door to pull her out. He finally convinces her to open the door and jump to him. He then materializes the TARDIS atop an office building and tries to cheer her up. The Doctor gives her a bioform damper ring to keep the roboforms from homing in on her, and then warns that Christmas is a dangerous time when aliens arrive on Earth. Donna, indifferent, tells him that she had a hangover during the last Christmas and doesn't remember anything about it. He still wonders why the alien mercenaries want her and how she appeared in the TARDIS. He scans her but determines that she has no special powers abilities. The Doctor asks where she works and Donna explains that she works at a security firm and met her husband-to-be, a HR director, six months ago when he brought her coffee. She wants the Doctor to take her to the reception and explain what happened, figuring everyone is heartbroken at her disappearance.
Everyone at the reception is dancing and drinking. When Donna arrives, they demand to know what happens. She bursts into fake tears to distract them and Lance embraces her while everyone applauds. While they talk, the Doctor calls Lance's security firm and taps into their database. He discovers that they're owned by Torchwood. He then talks with the cameraman and realizes that someone used huon energy to abduct Donna from the wedding. Realizing that the bio-damper won't work to shield huon particles, the Doctor runs to a window and sees the roboforms closing in. They activate a remote and the ornaments float away from the inside Christmas trees, and then start attacking the crowd, exploding.
The six roboforms enter the building and the Doctor feeds a sonic pulse through the loudspeaker system, causing them to overload. Examining them, he realizes that someone is using a remote signal to control them. He tries to locate the signal origin and realizes it's somewhere in the sky. As Donna runs after him, they're both unaware that a spidery figure is watching them from a webstar spaceship high in orbit.
The Doctor loses the signal and asks Donna and Lance to take him to their company, HC Clements. Once there, he accesses the database and realizes someone bought out the company after the Battle of Canary Wharf. He explains that huon energy particles were placed into Donna, and they acted like an opposing charge to the huon energy in the TARDIS, drawing her there. He realizes there's an undocumented hidden basement and prepares to go down on his own. Donna insists on going with him and drags Lance along. As they descend, the spidery figure chuckles in approval as the Bride approaches, and will act her key.
The trio explore the tunnels and the Doctor climbs up a shaft. He realizes they're beneath the Thames Flood Barrier, and Torchwood built the underground facility originally. Exploring further, the Doctor finds a room filled with liquid tanks. he explains that whoever took over discovered a way to extrude huon particles into liquid form. As the Doctor activates a small sample, Donna glows with energy. He explains that the particles are inert unless placed inside a living form. When Donna got married, her endorphin production increased and the particles inside of her triggered. The Doctor finally informs her that the particles are deadly but he promises to remove them from her.
The female voice of the spider creature echoes out as she broadcasts down over the intercom, and a secret panel in the wall slides aside. She explains that she has been hibernating on the edge of the universe awaiting the proper moment. As Lance runs away, an army of roboforms take the Doctor and Donna prisoner and take them to a huge shaft leading to the center of the Earth. The Doctor taunts the alien into teleporting down and she accepts. The Doctor realizes she's a Racnoss and she introduces herself as the Empress of her kind.
The Doctor realizes the Empress is the last of her kind and her race devoured whole planets during the Dark Age. As the Doctor distracts the Empress, Donna notices Lance come in with an axe. Donna adds to the effort and Lance approaches the Empress... and then laughs in triumph along with the Empress. Lance explains that he's been putting up with Donna all along and really despises her. The Doctor explains that Lance has been working with the Empress and gave her the huon particles in her coffee. He married her to keep her from running off. Lance plans to accept the Empress' reward of a chance to travel the universe an experience things no human will ever see. The Doctor tries to get the Empress to explain but she orders his death and calls in the roboforms. Confident, he notes that the attraction between Donna and the TARDIS is two-way and reverses the particle flow. The TARDIS materializes around them, protecting them from roboform gunfire. As the Doctor departs to Earth's ancient history, the Empress orders her roboforms to capture Lance as she realizes she needs another key.
The Doctor travels back to Earth's creation and shows a crying Donna to the doors. They look out on Earth as it forms, 4.6 billion years in the past. They see a Racnoss ship arrive and the floating matter starts drifting toward it, attracted by its gravity field.
In the present, the Empress force feeds the huon-water into Lance and plans to turn him into a key. She then uses the huon particles within him to pull the TARDIS back to the present. The Doctor manage to divert the TARDIS to a nearby corridor and tries to find a way through, unaware that a roboform has captured Donna. Thinking she's wandered off, the Doctor opens a panel only to find a roboform waiting for him.
The Empress has strung Donna up in her web next to Lance. She then purges the particles from both of them and release the huon energy down the shaft. The Empress explains that the energy will awaken the ship and her long-lost Racnoss children, which will spread across the Earth and consume all of its inhabitants. She summons her webstar and then mockingly cuts Lance free, letting him drop down the shaft to feed her children.
The people outside look up in wonder as a glowing (web)star appears in the sky. Their wonder turns to terror as the Empress has it fire on remote, devastating London.
As a roboform enters the chamber, the Empress addresses it as the Doctor, recognizing him through his disguise. He frees Donna and she swings free, misses the Doctor, and hits the wall. The Doctor gives the Empress one last chance to leave. He offers her and her children a planet to co-exist peacefully in the universe. She refuses and he tells her what happens next is her own doing. The Empress commands the roboforms to fire but the Doctor uses their own remote control to deactivate them. The Empress says her children will feast on him, but the Doctor explains that he's from Gallifrey and is a Time Lord, the race that wiped out the Racnoss. He tosses the explosive Christmas ornaments into the air and uses the remote control to blast open the corridors and let the water from the Thames in. The shaft is flooded, killing the Racnoss children, as the Doctor dispassionately looks on. Donna asks the Doctor to stop and he finally looks away and tells Donna to come with him.
As the Doctor and Donna climb to the surface and the Thames Barrier, the Doctor explains that the Empress is helpless now that she's used up all the huon energy. The Empress teleport to her webstar but a UNIT tank receives orders from a Mr. Saxon to open and fire and destroy the ship, killing the last of the Racnoss.
The Doctor uses the TARDIS to take Donna back to her family's flat. The Doctor apologizes for failing to save Lance but Donna admits that he deserved it. Using the TARDIS, the Doctor fires an energy burst into the clouds to make it snow and cheer her up. He wonders what she's going to do now and Donna says that she plans to travel a bit and learn a bit more of the big picture. The Doctor offers her the chance to come with her but Donna admits she can't take all the excitement. She invites him to join her family for Christmas dinner and he agrees, but then tries to slip away in the TARDIS. Donna yells for him and the Doctor pokes his head out to say that they may meet again if he's lucky. Donna makes him promise to find someone to travel with, insisting he needs someone to mellow him. As he tries to leave, Donna asks what his friend's name was and the Doctor finally says "Rose."