Shortly following the events in The Parting of the Ways, the TARDIS returns to Jackie Tyler's estate flat shortly before Christmas, greeted by both Jackie and Mickey Smith. However, the man accompanying Rose Tyler from the TARDIS is unfamiliar to the two of them, and seems to be in bad health. Rose explains, as they get him dressed in bedclothes left by Jackie's beau (who has a habit of leaving pieces of fruit in the pockets) and into bed, that he is Doctor who has just undergone regeneration. With little else to do while the Doctor recovers, Rose and Mickey go Christmas shopping but are attacked by masked Santas. They escape safely to the flat where they find a Christmas tree in the flat; as they realize that Jackie didn't buy it, the tree comes to life, spinning fast enough to destroy anything in its path. Rose grabs the Doctor's sonic screwdriver and places it in his hand, asking the still-comatose Doctor for help; instinctively, the Doctor awakes and uses the screwdriver to disintegrate the tree, and recognizes that it was being controlled by more Santas outside the flat, though they transmat away before anyone can stop them. The Doctor collapses again, telling Rose that he is still regenerating and that the energy of his regeneration is luring the Santas, and whomever controls them, here, and he is put back to bed by the others.
Meanwhile, the "Guinevere One" space probe, launched under a United Kingdom program by Prime Minister Harriet Jones, is about to land on Mars, with a live television broadcast from the probe once it lands. However, unknowingly, the probe is swallowed by a giant spacecraft that makes its way silently to earth. As the broadcast begins, the audience is surprised to find an alien face staring back at them. Harriet, along with the probe's team, learn that the broadcast came not from Mars but 5000 miles above Earth. The alien speech is eventually translated, and the aliens are revealed to be Sycorax, and they are claiming the planet as their own, demanding surrender or "they" will die. Harriet sends a response back, telling them that she will not allow it and that they are armed, but the Sycorax ignore her, and instead activate a device that causes nearly one third of the world population to fall under their control, leading these people to stand precariously at the edges of the highest roofs they can find, as if ready to jump. The probe team learns that the commonality of those under the Sycorax control is that they all share the same blood type as a sample that was included on the probe. Realising they are overwhelmed, Harriet pleads through the television broadcast for the Doctor's help. Shortly after this, she along with members of the probe team are transmatted to the Sycorax ship, where they demand that either half the world's population be sold to the Sycorax as slaves, or those standing on the roofs will be told to jump and will die.
Rose and Mickey, who have been aware of what has occurred through Mickey's hacking skills, move the Doctor back to the safety of the TARDIS as the Sycorax ship takes position over London. The TARDIS is transmatted to the spaceship, unbeknownst to Rose and Mickey, and thus when they leave it, they are panicked to find themselves facing the Sycorax, resulting in a container of tea being spilt into the underbelly of the TARDIS control room near where the unconscious Doctor is laying and causing some sparks to form. Rose attempts to bluff the Sycorax in a manner similar to what she had seen the Doctor use before with the Nestene Consciousness, but they see through her claims. However, Rose's deception has allowed the Doctor to fully recover, thanks to the spilled tea, and he emerges from the TARDIS, fully regenerated. Asking the Sycorax to wait, the Doctor goes about greeting everyone, and discovers and disables the device that was controlling the humans by blood, which can only hypnotize humans but cannot send them to their death; the Sycorax threat was a bluff.
The Doctor challenges the Sycorax leader to a one-on-one battle over the fate of earth, which the leader agrees to. They sword fight, first inside the ship, then moving out onto its wing, with the Doctor appearing outmatched, and at one point, having his hand cut clean off. The Sycorax believes he has won, but the Doctor notes that as he is still within the first fifteen hours of regeneration, his body has incredible regenerative powers, and he is able to regrow his lost hand and take the advantage to defeat the leader. At sword point, he gets the leader to vow to never return to Earth again, and begins to return with the rest of the group inside the ship. However, the Sycorax leader attempts to attack the Doctor from behind; the Doctor throws a fruit found in the bedclothes he's wearing at a control button that causes the area of the ship's wing to disappear, dropping the Sycorax leader to Earth.
The Doctor returns all the humans back to Earth and watch as the Sycorax ship starts to move away, warned off by the Doctor to never return again. Harriet receives a call and learns that "Torchwood is ready", and subsequently orders its activation. To the Doctor's horror, five powerful energy beams from across London converge on the Sycorax ship and destroy it. Harriet justifies it as needed to be able to defend the Earth from other hostile aliens when the Doctor is not there. The Doctor claims he can destroy her with six simple words, and after she walks off he asks her aide "Don't you think she looks tired?".
That evening, as snow (actually the ash from the destroyed ship) falls on London, the Doctor selects his new outfit from the TARDIS wardrobe, and joins Rose, Jackie, and Mickey for Christmas dinner, and watch Harriet Jones on the television, fending off rumors about her ill-health and a pending vote of no confidence in the House of Commons. The Doctor and Rose then get ready to set off again for more travels across space and time.
Doctor David Tennant (Tenth Doctor)
Companion Billie Piper (Rose Tyler)
Camille Coduri – Jackie Tyler
Noel Clarke – Mickey Smith
Penelope Wilton – Harriet Jones
Adam Garcia – Alex
Daniel Evans – Daniel Llewellyn
Sean Gilder – Sycorax Leader
Anita Briem – Sally
Chu Omambala – Major Blake
Sian McDowell – Sandra
Paul Anderson – Jason
Cathy Murphy – Mum
Sean Carlsen – Policeman
Jason Mohammed – Newsreader 1
Sagar Arya – Newsreader 2
Lachele Carl – Newsreader 3
Writer Russell T. Davies
Director James Hawes
Script editor Helen Raynor
Producer Phil Collinson
Executive producer(s) Russell T. Davies
Production code 2.X
Series Series 2
Length 60 minutes
Originally broadcast December 25, 2005