Rose refuses to send the Doctor to a hospital, as she fears what the authorities will do if they realise that he isn’t human. Jackie puts him up in the guest room, dressing him in a pair of men’s pyjamas and a robe; Rose wonders where they came from, and her mother explains that Howard from the market has been dropping by. The Doctor’s two hearts are beating steadily, but Rose is still shaken by his transformation and isn’t sure whether he’s just changed his appearance or his whole personality. Neither she nor Jackie notice when the Doctor quietly exhales a cloud of glowing yellow energy, which drifts out of the flat and up into space...

Rose notices Harriet Jones on TV, and learns that the MP for Flydale North has been elected Prime Minister; already, under her guidance, this is being called Britain’s Golden Age. British scientists led by the enthusiastic Daniel Llewellyn have constructed an unmanned space probe, Guinevere One, which is even now descending towards Mars to take pictures of the planet’s surface. At least, that’s the plan -- but on its way down, the probe encounters an alien spacecraft like a giant asteroid, and is sucked inside...

As night falls, Mickey and Rose go shopping for food. Rose realises that she’s irritating Mickey by talking about the TARDIS and the Doctor all the time, and promises to give Mickey one night without weirdness. However, as they pass a brass band of Santa Clauses playing God Rest Ye Merry Gentlemen, the Santas turn to follow them, and just as Rose realises that they’re wearing metal masks to hide their faces, the Santas lower their instruments and use them to shoot at Rose and Mickey. As the crowd scatters, a blast from a bazooka/tuba hits the tree in the middle of the street, which topples and crushes one of the Santas beneath it. Rose and Mickey rush to safety and catch a taxi back to Jackie’s flat, realising that the Santas were after them because they’re housing the Doctor. Unfortunately, Jackie’s on the phone with her friend Beth, and Rose is unable to get through to warn her of the danger.

Rose and Mickey rush back to the flat, and as they try to warn the bewildered Jackie to get out, Rose notices that her tatty white artificial tree has been replaced by a lush green one. Jackie claims that it was just delivered, and that she thought Rose and Mickey had bought it; however, the tree now lights up and begins to spin towards them, its artificial needles chewing the room’s furniture to splinters. Rose, Mickey and Jackie retreat into the guest bedroom and barricade the door, but the tree chews its way through regardless. Desperate, Rose places the sonic screwdriver in the Doctor’s hand and begs him to help her -- and he snaps awake, points the sonic screwdriver at the tree, and blows it apart in a cloud of flame.

The Doctor rises from the bed and steps out onto the balcony, where the three Santas are watching from the courtyard. When the Doctor points the sonic screwdriver at them, they teleport away in blurs of blue light. The Doctor refers to them as “pilot fish,” but before he can explain, he begins to convulse; Rose has woken him too early, with his regeneration cycle still incomplete, and he’s suffering from a neural implosion. He coughs out another cloud of yellow light and explains that his body is bursting with regenerative energy; the pilot fish sensed this and came here to destroy his defenders and kidnap him so they could run their batteries off his body. The Doctor is momentarily distracted when he finds an apple in his dressing gown, and Rose is startled when Jackie explains that Howard gets hungry in his sleep. Nevertheless, before fainting again, the Doctor warns the others that the arrival of the pilot fish means that something else, something worse, is on its way.

Rose and Jackie get the Doctor settled down again in the guest room, but his condition has deteriorated and one of his hearts has stopped beating. While Mickey looks up “pilot fish” on his laptop, Jackie turns on the news, and sees a somewhat worried Daniel Llewellyn assuring the press that, despite a temporary loss of contact with Guinevere One, they should be receiving the first live transmissions from the surface of Mars at any moment. Meanwhile, Mickey learns that pilot fish are small scavengers that swim alongside bigger fish like sharks -- and at that moment, the transmission from Guinevere One appears live on television, revealing the snarling, bone-like face of an alien.

While the media go into a frenzy of speculation, Daniel Llewellyn is transported to a secret UNIT HQ bunker beneath the Tower of London. In the ops room, Major Blake introduces Llewellyn to Harriet Jones and her aide, Alex; they have released a cover story claiming that students have hijacked the transmission and staged a prank, but the story won’t hold for long. A technician named Sally Jacobs reports that the probe was in fact broadcasting from 5,000 miles above the Martian surface, which presumably means that it’s aboard an alien ship; the aliens weren’t Martians, but another species that happened across the probe. UNIT satellites then pick up the ship itself, moving towards Earth -- very fast. The aliens send another transmission via the probe, and Blake orders Alex to run the translation software.

In Jackie’s flat, Mickey hacks into the military computers, as he’s done once before, and finds them tracking the approaching ship. He also picks up the alien transmission, but Rose is disturbed to find that she can’t understand the alien language. Normally, the TARDIS automatically translates for her, and if it isn’t doing so now, that must mean that part of it is broken... and that part may be the Doctor.

The American President contacts UNIT, demanding to be put in charge, but Jones flatly refuses to let him turn this incident into a war. As the alien ship approaches Earth, Jones asks Major Blake if there’s been any sign of the Doctor; there hasn’t, and she thus orders Blake to get in touch with the top-secret organisation known as Torchwood. Blake is reluctant to involve them, but Jones promises to take full responsibility. Alex then reports that the aliens’ message has been translated; they apparently call themselves the Sycorax, and are claiming that “they” will die unless humanity surrenders, though it’s not clear whom the Sycorax mean by “they”. Jones tells Alex to reply that today is a day of peace, and that humanity greets them on those terms... and that the planet is prepared to defend itself by force if necessary.

As dawn breaks, the Sycorax respond to Jones’ reply. On the main screen at UNIT ops, the Sycorax leader raises his hand, which glows blue, as if he’s casting a spell. The same blue light shimmers around the heads of a number of people in the ops room, including Sally Jacobs, and all those affected fall into a trance and walk out of the room. Blake and Llewellyn follow them upstairs to the roof, where the entranced victims line up on the edge, as if about to jump. The same is happening all across the country; on the Powell Estate, Rose and Mickey emerge from the flat to find their neighbour, Sandra, trying to get a man named Alex to stop walking. Out on the streets, a frantic mother tries to stop her husband and children from walking away with the crowd, and a stunned policeman watches as hundreds of people line up on the roofs of every high-rise building in the city. Soon word reaches UNIT HQ that the same thing has happened all across the world; a full third of the planet’s population, about two billion people, is standing on the edge of roofs, preparing to jump. The meaning of the Sycorax’s ultimatum is now clear.

Alex reports that the entranced people are often direct relations, but not husbands and wives. This gives Llewellyn an idea, and when he checks the medical records of the affected UNIT staff, he discovers that they all have A-positive blood. Llewellyn reveals that he placed a plaque on Guinevere One, a message of hope to the Universe, containing maps, music -- and a vial of A-positive blood, which it seems that the Sycorax are using to control their victims. Torchwood is not yet ready to act, and Jones thus goes on TV to address the nation. She informs them that the Queen’s holiday speech has been cancelled, since the royal family is up on the roof, and urges her constituents to remain calm... and she then makes a personal plea for the Doctor, if he’s watching this, to come and help them.

Rose bursts into tears upon hearing Jones’ plea, as the Doctor is still lying unconscious in the guest bedroom -- and unlike the Doctor she knew, this new man isn’t going to pop up and save them all. As she cries, there is a thundering crash, and windows shatter all across the city of London as the Sycorax ship enters the atmosphere. Soon the ship is hovering above the city like a gigantic floating mountain. Rose orders Mickey and Jackie to help her carry the Doctor into the TARDIS; she doesn’t know how to stop the aliens, and all she can think to do is hide. Jackie collects bags of food, but is forced to make two trips to carry it all while Rose and Mickey carry the unconscious Doctor into the console room.

The Sycorax contact UNIT again, demanding that the planet’s tribal leader step forward. Jones addresses them on behalf of humanity, and she, Alex, Blake and Llewellyn are teleported aboard the Sycorax ship into a vast, cavernous space filled with snarling Sycorax. The Sycorax leader steps forward and removes his helmet -- to reveal an even more hideous face beneath, like bone wrapped around raw muscle. Llewellyn takes responsibility for supplying the Sycorax with the blood they’re using to control the Earth’s population, and he steps forward and begs the Sycorax leader to show mercy and compassion to the human race as it takes its first steps to the stars. Instead, the leader strikes Llewellyn down with an electric lash, frying him to a skeleton in seconds -- and does the same to Blake when the appalled Major protests at this treatment of a prisoner. The leader then repeats its ultimatum to the shaken Jones, and Alex translates via his palm-pad, telling her that the Sycorax are demanding half the world’s population as slaves. If Jones refuses to surrender, the Sycorax will unleash the final curse and force one-third of the world’s population to jump to their deaths.

As Jackie returns to the flat for more food, Mickey and Rose study the TARDIS console. Rose believes that the memory of how to fly the TARDIS has been erased from her mind as a safeguard; however, Mickey recalls that one of the console screens can pick up TV signals. He and Rose try to activate it, but instead trigger a signal -- which is detected aboard the Sycorax ship. The Sycorax leader accuses Jones of hiding alien technology on her planet and orders his people to bring it on board; down below, the TARDIS is teleported away seconds before Jackie can step back inside. Inside, Mickey pours himself a cup of tea from Jackie’s thermos, and Rose steps outside to find out what’s keeping her mother -- and is captured by the Sycorax. Hearing her scream, Mickey drops the thermos and rushes to investigate; when he finds himself surrounded by Sycorax, he doesn’t have time to get back inside, but he does manage to shut the door, keeping the Sycorax out. Meanwhile, tea spills out of the thermos, through the floor of the console room and over the controls beneath, which begin to spark and fizz. Sweet-smelling steam wreathes the Doctor’s head, and he begins to stir...

Harriet Jones is delighted to see Rose again, but Rose must then break the bad news that the Doctor won’t be there to help them this time. The Sycorax leader, assuming that Rose is in charge of the blue box, demands that she speak, and despite Mickey’s and Harriet’s protests, Rose steps up to try to fill the Doctor’s shoes. She demands that the Sycorax leave the Earth in peace, citing the authority of every alien race she can think of -- but it’s clear that she’s bluffing, and the Sycorax leader bursts out laughing and threatens to kill her and take this planet by force. But as Alex translates the leader’s threats, the humans slowly realise that they can hear the Sycorax leader speaking English, even though he hasn’t actually stopped speaking Sycoraxic. The TARDIS has begun to translate for them again, and as Rose turns to look, the doors open and the Doctor steps out, his synapses healed by the effect of inhaling the free radicals and tannin in Jackie’s tea.

The Sycorax leader lashes his electric whip at the Doctor, but the Doctor grabs it out of mid-air, tosses it aside and tells the leader to wait. He’s got more important considerations; he wants Rose to tell him what colour his hair is. Disappointed to learn that he’s not ginger, he snaps at Rose for giving up on him, but then realises that this was rather rude of him and worries that his new personality may be an unpleasant one. Jones is bewildered, wondering what happened to the man she knew as the Doctor, but the new Doctor reminds her of the time they spent trapped in 10 Downing Street, and how her greatest fear was that her mother would be left alone. He’s still the same man, in a way -- but when the furious Sycorax leader demands to know who he is, the Doctor admits that he doesn’t know yet. He does, however, notice the big threatening button on the blood-control matrix, and when he works out what it’s for, he decides that he’s the kind of man who likes to press big threatening buttons. Before the others can stop him, this is just what he’s done.

Appalled, Alex tells the Doctor that he’s just killed two billion people -- but the Sycorax leader, caught out, claims that they have magnanimously decided to let their victims live. The Doctor scoffs and reveals that blood control is just a cheap form of hypnotic conditioning; there are limits to what it can do, and just as it’s impossible to hypnotise somebody to death, it was impossible for the Sycorax to make their victims leap to their deaths. Down on Earth, two billion confused people have just woken up and stumbled back out of danger. Furious, the Sycorax leader threatens to call in the rest of the armada and invade the Earth by force. The Doctor tries to reason with him by citing the wonderful things that humanity can accomplish, but then realises that he’s quoting a line from The Lion King, and instead doffs his borrowed bathrobe and offers to challenge the Sycorax leader to a duel. Whatever kind of person he is, he still stands as Earth’s champion, and he and the Sycorax leader will do battle to determine who wins the Earth.

The Doctor seizes a sword from a nearby Sycorax, and he and the leader begin to fight; however, the Doctor is still finding his feet, and doesn’t do terribly well at first. Deciding to take the fight out into the open, he punches a control on the wall and takes the fight out onto the surface of the ship’s hull, hovering miles above the city. Rose is unable to interfere for fear of voiding the contest, and must watch helplessly as the Sycorax leader knocks the Doctor down -- and chops off his right hand, sword and all. But as the Sycorax leader steps back, apparently triumphant, the Doctor reveals that he’s still within the first 15 hours of his regeneration cycle -- and that he still has enough residual cellular energy to grow a new right hand. Rose tosses the Doctor a sword, and this time, the Doctor defeats the Sycorax leader. With his sword to the leader’s throat, the Doctor orders him to swear on the blood of his species that he will leave this planet in peace. The Sycorax leader reluctantly does so.

Satisfied, the Doctor turns back to his friends, and the delighted Rose hands him his bathrobe, the Doctor finds a satsuma in the robe’s pocket -- another of Howard’s snacks. As he muses on the appropriateness of finding a satsuma at the end of Christmas, the enraged Sycorax leader picks up his sword and charges the Doctor from behind -- and, without looking back, the Doctor flings the satsuma at a control on the hull, causing the hull to swivel open beneath the leader’s feet and drop him, screaming, down to the city miles below. The new Doctor is the kind of person who doesn’t give second chances. The Doctor and his friends return to the cavern inside the Sycorax ship, where the Doctor addresses the assembled hordes, telling them to go back out into space with a warning: the Earth is defended.

The Sycorax teleport the Doctor and his friends back to Bloxham Road, just around the corner from the Powell Estate, and then turn and retreat, leaving the Earth in peace. Jones is delighted, and accepts that this Doctor is indeed the same man she once knew. However, she’s concerned that there might be more aliens like the Sycorax out there, and the Doctor confirms that this is the case; there are thousands of alien species out in the galaxy, and as the people of Earth venture out into space, they’re beginning to draw attention to themselves. Jackie then arrives, and as the Doctor crosses to greet her and thank her for the tea, Alex receives a call from Torchwood and informs Jones that they’re ready to go. As the Doctor and his friends celebrate their victory, the Prime Minister hesitates for a moment -- and then gives the order to fire. Moments later, violent green bursts of energy spear up from London and blow the retreating Sycorax ship to bits.

Jones informs the appalled Doctor that this was a defence system reverse-engineered from an alien craft that crashed on Earth 10 years ago. The Doctor isn’t always there to save them, and today, Jones saw two men die before her eyes while the Doctor was sleeping; humanity has to be ready to defend itself. The Doctor is sickened to learn that the architect of Britain’s Golden Age is willing to shoot a retreating enemy in the back, and realises that the real monsters here are the human race. He tells Jones that he would have stopped her if he’d known what she intended, and when she points out that this makes him another alien threat, he angrily tells her that he can bring down her government with just six words. She tries not to show that this rattles her, but the Doctor simply walks over to Alex and whispers in his ear: “Don’t you think she looks tired?” That done, he walks off without a look back, as the increasingly frantic Harriet demands to know what he said and Alex looks at her, slightly worried.

While Rose, Mickey and Jackie prepare turkey dinner in Jackie’s flat, the Doctor returns to the TARDIS and selects new clothing for his new incarnation: an understated brown suit, tie and coat, and casual sneakers. He then joins his friends for Christmas dinner, and as they eat, the news comes on television; rumours of Harriet Jones’ ill health are spreading, and despite her attempts to assure the media that she’s feeling just fine, it appears that the Opposition may call for a vote of no confidence in her government. Jackie’s friend Beth then calls them outside, where they see thick, fluffy snow falling from a sky full of shooting stars -- or so it appears until the Doctor reveals that it’s actually ash from the Sycorax ship burning up in the atmosphere. After today, the people of Earth will have to accept the existence of alien life. As for the Doctor, he’s ready to set off on his travels again, and despite her earlier concerns, Rose is ready to go with him. The disappointed Mickey realises that she’s still not ready to settle down, but as always, he and Jackie must accept her decision. Together, the new Doctor and Rose will set off into space to see what wonders are out there...