On Earth on Christmas Eve, Jackie Tyler is decorating her Christmas tree at home and considers the present that Rose left for her.
Mickey is working at the auto shop when he hears the sound of the TARDIS arriving. Jackie hears the same noise and they both run outside. They see the TARDIS materialize and then come smashing down, hitting everything in its path. It comes to a halt and the newest Doctor, wearing his predecessor's clothing, staggers out. He recognizes Jackie and Mickey, who have no idea who he is. He wishes them Merry Christmas and then collapses, and Rose emerges from the TARDIS. She explains that this is the new Doctor.
Jackie and Rose take the unconscious Doctor up to their flat and Rose refuses to let him go to the hospital since he'd be dissected. Rose confirms that the Doctor's two hearts are beating, They leave him to recuperate, unaware that a brief burst of golden energy leaves his body and drifts up into space.
Jackie wonders what regeneration means for the Doctor, and Rose snaps at her, admitting that she thought she knew the Doctor but now he's changed. When Rose wonders where Jackie got the pajamas she put on the Doctor, Jackie admits she's dating a new man. Rose notices Harriet Jones, the new Prime Minister, doing a newscast on the television. She's defending her decision to launch the Guinevere One
space probe, which is now 50 million miles away and descending toward Mars. In space the probe bumps into a large stone-like object, which sucks it in.
Later, Rose and Mickey go walking and she borrows some money for him for Christmas the next day. Mickey is less than thrilled as she talks about the TARDIS, and tells her to just let it be Christmas with the two of them together. She promises to give it a try and they pass by some street Santas playing instruments. Rose notices the Santas fixating on her, and then opening fire with weapons concealed in their instruments. Mickey and Rose run for cover as one of them brings down a Christmas tree on itself, knocking its mask loose. The two of them take refuge in a taxi and Rose tries to call to warn Jackie, but she's on the phone. Rose figures that the Santas are after them because they're connected to the Doctor.
At the flat, more energy emerges from the Doctor.
Rose and Mickey arrive at the flat and hang up Jackie's phone. They figure they can go to Cousin Mo but Jackie doesn't understand. Rose tries to explain and notices that Jackie has a new tree. Jackie explains that someone delivered it, but Rose has no idea where it came from. The Christmas tree starts spinning its branches and moving toward them, shattering everything in its path. Rose runs for the Doctor while Mickey tries to hold off the tree with a chair. They barricade the door to the Doctor's room but the tree starts to break through. Rose grabs the sonic screwdriver and puts it in the Doctor's hand, and then begs him to help. He wakes up, shatters the tree, and immediately wonders who is controlling it. He goes outside and sees the Santas down below. When the Doctor raises his sonic screwdriver at them, they back away and then teleport into the sky. He explains that they're just pilot fish, and then collapses in pain. The Doctor warns them that he's still regenerating and bursting with energy. The pilot fish could smell his regenerative energy a million miles away, so they came to Earth to steal him as an everlasting battery. The Doctor warns them that his brain is collapsing and they don't have much time until something else is coming.
Back inside, Rose treats the Doctor and realizes one of his hearts has stopped. They watch the news as Daniel Llewellyn of the Space Centre reports that Guinevere One
disappeared off the scope for a few minutes, but is now back online and ready to transmit the first pictures from Mars. Mickey looks up information on pilot fish and realizes that they accompany much larger, vicious creatures. Jackie watches the broadcast and they turn to see an alien face on the screen. It snarls at the camera, and newscasters across the world inform the people that life on other planets has been confirmed.
Major Blake brings Llewellyn to UNIT HQ in London and a meeting with Harriet Jones. She has already established a cover story with the assistance of Alex, her right-hand man. She admits that the transmission was genuine and it's a species of alien they haven't encountered before. One of the technicians, Mrs. Jacobs confirms that the transmission came from a point 5,000 miles above Mars. They realize that the probe is aboard an alien ship, and the ship is moving toward Earth.
Mickey hacks into the military computers and detects the alien shipping heading for Earth. He and Rose watch as the aliens broadcast to UNIT in an unknown language. Rose doesn't understand it despite the TARDIS translation facility, and they realize something serious is wrong with the Doctor.
At UNIT HQ, Alex determines that the ship is five hours away from Earth, and that it will take time to translate the language. As the world waits, Harriet confirms that there has been no sign of the Doctor. She tells Blake to that they need Torchwood, and takes full responsibility for getting them ready. Major Blake hastens to obey. Alex runs a partial translation and confirms that the aliens call themselves the Sycorax, and are claiming the people of Earth and the planet itself as their property. They state that if Earth doesn't surrender, "they" will die. Harriet sends an offering of peace but warns that they are armed and will not surrender.
As they watch over the Doctor, Rose insists the old Doctor wouldn't simply lie in bed. Mickey figures that she really loves him, and holds her in comfort.
The next morning, the Sycorax respond. On the viewscreen, their leader extends a hand glowing with energy. Jacobs and several other staff members turn into mindless automatons and march outside. Across the planet, others fall under the same spell. Rose and Mickey look outside and see hundreds of men, women, and children, all heading in the same direction: toward any high-rise building. They all march to the top to the very edge, and no one is able to get through to them. Reports come in of people doing the same thing all over the world, a total of one third of Earth's population.
The authorities realize that the Sycorax will command them to jump if Earth doesn't surrender. Alex confirms that everyone entranced is related, and Llewellyn checks the space probe's files. Meanwhile, Harriet tells Major Blake to get whatever they need from Torchwood. Alex and the others confirm that everyone affected has A positive blood. Llewellyn admits that he placed a plaque in the space probe, and it included a blood sample.
Harriet goes on the air and asks for the Doctor to come to their aid. Rose watches the broadcast and then turns away and cries as she looks at the unconscious Doctor and realizes that he's left her.
A sonic wave shatters windows everywhere as the spaceship hits the atmosphere. Everyone in London without A positive blood looks to the sky and sees the enormous Sycorax spaceship arrive overhead. Rose realizes that she's helpless and has Mickey and Jackie gather everything they can to take refuge in the TARDIS.
The Sycorax call upon Earth's leader to come aboard. They teleport Harriet, Blake, Llewellyn, and Alex aboard their ship, into a vast arena filled with thousands of the aliens. The Sycorax Leader removes his helmet to reveal an alien visage. He calls upon them to surrender or he will order the A positive humans to jump. Llewellyn steps forward, insisting that it's his responsibility, and tries to convince the Sycorax Leader that they are children by comparison, who need help. The Sycorax Leader transforms him into a skeleton with an energy whip, and does the same to Major Blake when he objects. Harriet steps forward but the Sycorax Leader is unimpressed and repeats his demand for surrender. She asks why surrender would be better, and he explains that they will sell half into slavery, but the alternative is the death of one third.
Mickey, Rose, and Jackie get the Doctor into the TARDIS, and Rose admits she can't pilot it since the knowledge has been wiped from her memory. As Jackie goes to get food, they realize they can use the TARDIS screen to tune into the alien broadcasts. Aboard the ship, the Sycorax detect the transmission and accuse Harriet of concealing advanced technology. The teleport the TARDIS aboard their ship, leaving Jackie behind.
Aboard the TARDIS, Rose finally goes outside and realizes where they are. The Sycorax grab her immediately and Mickey runs outside, dropping a tea thermos. He realizes what is happening and closes the door before the Sycorax can get inside. Meanwhile, the tea leaks out of the thermos and into the TARDIS floor. Outside, Rose admits that the Doctor is no longer with them.
Smoke from the components in the TARDIS rise up and moiré regenerative energy escapes from the Doctor.
The Sycorax Leader believes that Rose speaks for the planet since she has the TARDIS. Rose tries to bluff, commanding them to leave, but the Sycorax are merely amused. They order Rose's death and stop Harriet and Alex from intervening. The Sycorax Leader finally speaks English, and Rose realizes they can understand him because the TARDIS is now translating again. She turns to the TARDIS and the Doctor emerges.
The Sycorax Leader attacks the Doctor with his energy whip, but he casually disarms him and snaps the whip. He greets everyone and explains that the super-heated tea healed his synapses. He introduces himself to Harriet as the same man with a new face, and quickly confirms he is who he says. The Sycorax Leader demands to know who he is, and the Doctor admits that he's not sure beyond the fact that he's the Doctor. He does notice the big threatening button the Sycorax used to control the A positive humans, and realizes it's a control matrix. The Doctor finds the A positive human blood inside and realizes what it means. He presses the button and on Earth, everyone is freed from the spell.
The Sycorax Leader claims that he let the humans live, but the Doctor explains that it was merely a form of hypnosis but it can't drive anyone to kill themselves. Unimpressed, the Sycorax Leader threatens to summon an armada to destroy the planet. The Doctor draws the Sycorax Leader's sword and challenges him to ritual combat. The Sycorax Leader accepts and the two begin their fight. The Doctor runs out onto the hull of the ship overlooking London and warns the others not to interfere so the challenge isn't invalidated.
The Sycorax Leader knocks the Doctor down and then cuts off is right hand. The Doctor realizes who he is, stands up, and explains that since he is still within his regenerative cycle, he has enough energy to regenerate his hand. Rose tosses him another sword and the Doctor renews his attack, stabbing the Sycorax Leader. He offers to spare his life if he leaves the planet and never returns. The Sycorax Leader agrees, swearing on the blood of its species. However, once the Doctor turns his back, gets the bathrobe back, and finds an apple in the pocket. The Sycorax Leader grabs a sword and lunges at him, and the Doctor throws the apple, triggering a button that opens a panel and drops his opponent to his death far below. The Doctor notes that as a new man, he doesn't offer second chances.
Back in the ship, the Doctor tells the other Sycorax that he has won, and to warn others that Earth is defended. they are all teleported down to London with the TARDIS. They watch as the Sycorax ship departs. Harriet thanks the Doctor and they embrace. She wonders if there are other aliens out there, and he admits that there are hundreds, and Earth is drawing attention to itself. Jackie runs up and is impressed to meet the Prime Minister, and they all share a hug. Meanwhile, Alex gets a message from Torchwood saying that they're waiting. Harriet looks at the Doctor, and then tells Torchwood to fire. Somewhere in London, a vast series of energy beams shoot into the sky and blast the Sycorax ship into a million pieces. The Doctor realizes what happened and accuses her of murder. Harriet insists that it is defense, and that they adopted the weapon from alien technology found on Earth. She insists that Earth has to protect itself and the Doctor isn't always there. The Doctor admits that he should have told the Sycorax to run from the human monsters. He vows to bring her down with just six words, and then whispers to Alex, "Don't you think she looks tired?" The Doctor then walks off with the others and an increasingly harried Harriet demands to know what he said.
In the TARDIS wardrobe, the Doctor chooses his newest clothing while Mickey and the Tylers prepare Christmas dinner. He comes in and they celebrate together, and then watch a newscast where the press asks an unnerved Harriet if she's unfit for office because of health concerns. They go outside and discover that it's snowing on Christmas day. However, the Doctor explains that it's ash from the Sycorax spaceship. He figures that everything is new now that everyone has witnessed aliens. The Doctor prepares to leave and wonders if Rose is coming with him. She insists that she is despite the fact that he's changed, and she wonders if he doesn't want her because he's changed. Mickey realizes that she's never going to stay. Rose wonders where they're going to go first and the Doctor points out their destination among the stars.