The Doctor in his TARDIS as it orbits a supernova, finishes his farewells to Rose Tyler, when suddenly a bride appears in the control room. Both are astounded by this event. Donna, the bride, demands the Doctor return her to the church in Chiswick where she was just in the middle of her wedding ceremony before appearing on the TARDIS. The Doctor tries to return there, but ends up near Oxford Street. Donna storms out of the TARDIS and tries to contact her family through a public telephone, while the Doctor wonders how she could have got aboard. He then notices familiar masked Santas, recognising them as the robotic scavengers from the previous year's Christmas, levelling their weapons disguised as instruments at him; he distracts them by using his sonic screwdriver on a nearby ATM to make it spit out money and causing a crazed rush from the nearby crowd, and then goes off in search of Donna. He finds she has managed to collect some money and is taking off in a cab, though its driver is another one of the robotic Santas. Donna quickly realizes that her cab is not taking her to the church, and discovers that the Doctor has managed to engage the TARDIS in pursuit of the cab along a highway and is able to rescue her. However, such an endeavour has put a strain on the TARDIS, and he cannot use it for some time. The Doctor gives Donna a ring to prevent the Santas from tracking her and tries to learn more about her, learning that she works at a security firm called H.C. Clements, and that that is where she met her husband-to-be, Lance. The Doctor takes Donna to her reception, to the relief of Lance, her family and their friends. The Doctor learns that H.C. Clements is owned by the Torchwood Institute, and that through footage taken by the photographer at the wedding earlier, that Donna was turned into Huon particles, a source of energy that hasn't existed for billions of years, and cannot be masked by the ring he gave her earlier. He finds the reception hall surrounded by the Santas, and that they have rigged a Christmas tree's ornaments to fly off explosively at the crowd. Using the sound system at the reception and his sonic screwdriver, the Doctor is able to shake apart the robots, and trace the source of their control to star-shaped spaceship hanging above the city, but shortly loses its signal.
The Doctor asks Lance to take her and Donna to H.C. Clements, learning that after Torchwood One's dissolution in the Battle of Canary Wharf, someone else took control of the company. The Doctor discovers a basement level not on the floor plans and the three travel to it, finding themselves in a long tunnel that eventually leads to the Thames Barrier. There, the Doctor discovers a laboratory where Huon particles have been manufactured and stored in liquid form. The Doctor determines that Donna was saturated in them, and due to the stress of her wedding day, causes the particles to catalyze and activate, pulling her into the TARDIS' own source of Huon particles. A large hole also extends across the room, the Doctor surmising that it was dug out by Torchwood's laser technology and extending to the centre of the Earth. As they explore, a half-humanoid, half-spider teleports into the lab, which the Doctor recognises being one of the Racnoss, a race thought wiped out billions of years ago by the Fledgling Empires during the Dark Times of the universe. The Racnoss calls itself the Empress, and has fashioned a large web above the pit, where the body of H.C. Clements still hangs. As the Doctor talks to the Empress, Lance sneaks around behind it with an axe, threatening to strike, but quickly reveals that he has been working with the Empress; he had spiked the coffee he gave to Donna every day with Huon particles as to allow them to mature in her, so that the Empress can use their activated energy to regain her ancient power. However, as the Doctor and Donna are targeted by more of the robotic agents under the Empress' control, the TARDIS materializes around them and they escape. The Empress is not thwarted as she knows exactly how to achieve the same result with Lance, and begins to force feed him the Huon liquid while trapping him in her web.
The Doctor takes the TARDIS to the creation of the Earth to learn why the Racnoss has dug into the core of the planet, and finds that a Racnoss spaceship is the actual core of Earth, the rest of the planet forming around it, and that if the Empress were to use the Huon particles, she would be able to reawaken those still on the ship. Armed with that knowledge, he and Donna return to a corridor by the laboratory, but Donna is immediately captured into the Empress' web, while the Doctor is held at gunpoint. The Empress begins to activate the Huon particles and, knowing that her fellow Racnoss will be hungry, severs Lance from the web dropping him into the pit. Meanwhile, the Empress' ship begins to descend towards the city, initially mistaken as a Christmas star before it starts firing on the city. The Doctor manages to sneak back into the laboratory and issues the Racnoss a final offer, to take her and her kind to a planet where they will not threaten anyone. She refuses, and the Doctor reveals himself as from Gallifrey, (his long-gone race having previously defeated the Racnoss). The Empress shrieks in fury, accusing the Time Lords of having murdered the Racnoss, but the Doctor simply tells her that what happens next is of her own doing, and using some of the explosive Christmas ornaments, bursts apart the walls and floods the room with water from the River Thames, drowning the infant Racnoss at the Earth's centre. The Doctor is able to escape with Donna into the TARDIS, while the Empress teleports back to her ship, moments before it is blown out of the sky by tanks under orders of "Mr Saxon".
The Doctor returns Donna home, but she is desolate having lost her job and her fiancé the same evening, and the Doctor uses a burst of energy from the TARDIS to make it snow in hopes to cheer her up, and offers her to join him on the TARDIS. She declines, but encourages him to find someone, recognizing that he had just lost someone himself. The Doctor tells her briefly about Rose, and then disappears into his TARDIS.
Doctor David Tennant (Tenth Doctor)
Companion Catherine Tate (Donna Noble)
Sarah Parish – Empress
Don Gilet – Lance Bennett
Howard Attfield – Geoff Noble
Jacqueline King – Sylvia Noble
Trevor Georges – Vicar
Rhodri Meilir – Rhodri
Krystal Archer – Nerys
Zafirah Boateng – Little Girl
Paul Kasey – Robot Santa
Writer Russell T. Davies
Director Euros Lyn
Script editor Simon Winstone
Producer Phil Collinson
Executive producer(s) Russell T. Davies
Production code 3.X
Series Series 3
Length 60 minutes
Originally broadcast 25 December 2006