Bride-to-be Donna Noble prepares to come down the aisle to her waiting husband, Lance Bennett, for her Christmas Eve wedding. However, halfway down the aisle she is surrounded in a golden glow, and then dissolves into a cloud of energy and drifts through the ceilingâ€¦ and appears in the TARDIS in mid-flight. Donna is not amused at being stolen from her wedding and blames the Doctor, demanding answers. She yanks open the doors and finds herself staring out into deep space. Shocked back to her senses, she realises the Doctor is an alien and that what she is seeing is real. The Doctor cannot figure out how she gained entrance to the TARDIS, and Donna demands that he return her to the church. She spots Rose's jacket and accuses him of having abducted other women. The Doctor gloomily admits that "she's gone".
The Doctor tries to get to the church but they materialise in an alleyway in the business districts. Donna tries to call for a taxi without success, but the Doctor gets one, however the driver lets them out when he realises that they don't have any money. The Doctor goes to an ATM to get cash with his sonic screwdriver, after using it to get Donna a call through to her mother. However, her mother's mobile phone is dead, and so Donna leaves a message.
The Doctor notices three roboform Santas approaching whilst Donna borrows money to get a cab. However, she doesn't notice that another Santa is driving the cab. The three roboforms prepare to open fire and the Doctor creates a distraction with the ATM, then goes to the TARDIS and gives chase to the cab, which has pulled on to the motorway.
Donna realises that the cab driver is up to no good while the Doctor closes in. The Doctor locks the TARDIS on course and tries to get Donna to open the door so she can jump. He opens the window and after a desperate pursuit, convinces her to trust him and jump across.
The TARDIS materialises atop a downtown building and the Doctor tries to cheer Donna up without much luck. He then gives her a bio-damper ring to keep the roboforms from homing in on her, and tries to explain the previous year's Christmas invasion. Donna doesn't remember it because of her hangover. The Doctor scans her for any disturbances without success, then asks where she works. Donna says she's a secretary at HC Clements, a security firm where she met her husband-to-be Lance, who brought her coffee. Donna decides to go to the reception and insists on taking the Doctor along to explain.
Everyone is dancing and having a good time at the reception, much to Donna's dismay. They didn't know what else to do and Donna can't explain, so she bursts into (fake) tears to get sympathy. The party continues on a better note and the Doctor borrows a mobile phone to try to tap into HC Clements' database. He discovers they're owned by Torchwood. Clearly uncomfortable with the happy couple, the Doctor chats with the cameraman and is shown Donna's disappearance in the Church. He recognises the golden glow as huon energy, which hasn't existed for millions of years. Realising that the bio-damper won't conceal huon particles, the Doctor runs outside and sees the roboforms closing in on all the exits. They stop outside and one of them uses a remote to activate the ornaments on the Christmas tree. They start moving around... then attack the guests, exploding. All six of the roboforms enter and the Doctor sends a sonic pulse over the loudspeakers, destroying them. The Doctor determines that the roboforms were operated by remote control, then takes one of their heads and runs off to pursue the signal. He looks up to the sky, unaware that they're being observed by a shadowy spider-like figure in a floating spaceship, a webstar.
The Doctor goes to HC Clements with Lance and Donna and discovers there's a secret floor. He takes the lift down and Donna insists on going with him, bringing a reluctant Lance with them. The spider-figure is pleased, wanting "the Bride" to come further in.
The Doctor, Lance, and Donna explore the tunnels underneath the building. The Doctor climbs up a shaft while Lance tries to convince Donna to leave, without success. The Doctor determines that they are in a base beneath the Thames Flood Barrier, and Torchwood secretly built it. They then discover a huon distillation facility, where Torchwood have put huon particles into liquid form, which were then put into Donna. The particles are inert and need to catalyse inside a living form, and the wedding accelerated Donna's endorphin production, making her the perfect "host" for the huon particles. Donna realises that the particles are dangerous and the Doctor promises to cure her.
A hidden wall panel slides aside and the voice of the spider-creature taunts them. Lance sneaks off while the Doctor comes in and finds a huge shaft drilled to the centre of the Earth. The Doctor taunts her into teleporting down and the spider-creature arrives in the complex. The Doctor recognises her as the Empress of Racnoss and realises she is the last of her kind. The Racnoss were carnivores from the "Dark Age" that consumed whole planets. Lance arrives with an axe and Donna tries to distract the Empress. Lance prepares to swing... then both he and the Empress laugh. Lance has been working with the Empress all along and has been dosing Donna with liquid particles in the coffee he gave her. The Doctor tries to get them to explain but the Empress orders in her remaining roboforms to attack. However, the Doctor reverses the huon particle attraction and the TARDIS materialises around them as a shield. The Doctor then dematerialises the ship and sets a course back to the beginning of Earth's creation. With Donna gone, the Empress orders her roboforms to grab Lance.
Donna is crying over Lance's deceit as they arrive at the beginning of Earth's creation. They open the doors and look out upon the proto-Earth, 4.6 billion years in the past. As they look on, a Racnoss ship arrives and starts attracting the floating matter.
In the present, the Empress force feeds Lance the huon particles to make a new "key." He then acts as a magnet, drawing the TARDIS back. The Doctor grabs the extrapolator while Lance is hauled up into the Empress' wife. The TARDIS materialises in her chamber but the Doctor redirects it to a nearby corridor. The Doctor tries to sneak past a security door but Donna wanders off and he opens the door to face an armed roboform.
The Empress has captured Donna and strung her up in the web next to Lance. She then directs the huon particles down the shaft to awaken the Racnoss ship and all of her children... reborn to feast on human flesh. The Empress drops Lance into the shaft to feed her awakening children, then summons her webstar which takes up position over London. She activates its weaponry and it fires beams of devastating energy across the city.
The Doctor enters the chamber disguised as a roboform but the Empress quickly recognises him. He frees Donna, who swings to (almost) safety before hitting a wall. The Doctor offers the Empress the chance to go to a planet and exist peacefully, but she refuses and orders her roboforms to fire. The Doctor, anticipating her, overrides them with the remote control the roboforms used earlier. He then reveals that he is a Time Lord, the race that destroyed the Racnoss. He tosses the Christmas ornaments up into the air and uses them to blast open the walls and flood the complex. Water and flame fill the building as the Doctor looks on remorselessly. He then gets Donna out while the Empress teleports up to her webstar. The Doctor and Donna climb to the top of the Thames Barrier, and the Doctor notes that the Empress has used up all her huon energy, and as such is defenceless. Tanks under the orders of Mr. Saxon open fire and destroy the webstar.
The Doctor takes Donna back to her family's flat and tries to cheer her up by making it snow. They wish each other a Merry Christmas and Donna resolves to travel a bit and see more of the planet. The Doctor suggests she come with him but she refuses, as she can't take the excitement and the Doctor's unique nature. She invites him to Christmas dinner with her family but he ducks into the TARDIS and tries to leave. She yells for him and he pokes his head out to say they may meet again. She makes him promise to find someone, although he claims he doesn't need anyone. He wishes her luck and finally speaks Rose's name aloud before departing.