Inside the TARDIS, the Doctor offers to let Rose choose their destination. She selects a hundred years in the future and he dismissively takes them there but is unimpressed with such a dull place. Rose suggests he do better and he cranks up the TARDIS controls and takes them ahead to the year 12,005. Rose laughs in challenge and the Doctor decides to really impress her. Taking the TARDIS far, far forward, they finally materialise and he usher her out into a viewing gallery with the windows shuttered close. The Doctor opens the shutters and reveals that they're on a space station in orbit above Earth... and the sun has begun to expand to envelop it. He explains that they're five billion years in the future, and about to see the end of the world.
The Doctor and Rose proceed to the reception area, and he explains that the National Trust has kept the sun from expanding but has run out of money so are letting nature takes its course. The rich and powerful from across the galaxy are assembling on the station, Platform One, to watch the planet's destruction. The Steward, a blue-skinned alien, confronts them but the Doctor uses a blank sheet of psychic paper to convince him they're invited guests. In the reception area, Rose meets friendly aliens for the first time, including the sponsor, the Face of Boe: an enormous head in a container. A traditional exchanging of gifts occurs. The hooded Adherents of the Repeated Meme offer metal spheres to everyone, including the Steward. Moxx of Balhoun spits on Rose, giving her his bodily saliva. The Doctor doesn't have a gift but improvises and gives Jabe, a tree woman from the Forests of Cheem, breath from his lung. Impressed, she flirts with him and the Doctor seems to respond.
The final guest to arrive is Lady Cassandra O’Brien, the Last Human. She has undergone so much plastic surgery that she has been reduced to nothing but a face on sheet of skin, while her brain is in an attached jar. Her attending surgeons constantly spray her with water to keep her from drying out. She offers an ostrich egg and a jukebox as gifts, and a startled Rose gives in to culture shock and runs out. The Doctor goes after her, unaware that Jabe is scanning him and recognises the species his life signs belong to. Rose goes to a gallery and watches the expanding a sun. A repair woman, Raffalo, comes in but doesn't say anything. Finally she explains that Rose has to give her permission to speak. She's surprised when Rose does so, and starts to open a maintenance hatch. She explains that her people are part of the Jaggit Brocade. Rose, who has no idea what she's talking about, finally realises what she's let herself into by traveling with a stranger places she has no knowledge of. She leaves and Raffalo gets the hatch open and enters the maintenance shaft. She finds one of the robot spiders there and at first believes it's simply a new maintenance robot model. However, other robot spiders arrive, scanning her and she tries to retreat. She almost succeeds only for them to catch her and drag her back in.
In his office, the Steward is notified that the TARDIS has been fond and irritably informs the guests by intercom that teleportation is strictly forbidden. The workers take the TARDIS away and give the Doctor a claim ticket.
The robot spiders are multiplying and spreading through Platform One. One finds Rose in the observation gallery and starts to approach her, but hastily retreats when the Doctor arrives. She talks about how the unfamiliar surroundings and aliens have taken her off guard, and asks who or what the Doctor is. He dodges the question and instead explains why everyone is speaking English. The Doctor explains that the TARDIS is telepathically altering her thoughts so she perceives other languages as English. Rose isn't thrilled that he's altering her brain without letting her know, and returns to the question of his identity. Angry, he refuses to answer and gives her the silent treatment. She tries to calm him down and then notices her cell phone doesn't have a signal. Eager to seise on the distraction, the Doctor attaches a new component and Rose discovers she can now call her mother Jackie in the 21st century. They make small talk and Rose tells her mother she's doing fine. Calmed someone by the tie to home, Rose hangs up but then realises that everything and everyone she knows have been gone for five billion years. They're interrupted when the station shudders and the Doctor realises something is wrong.
In his office, the Steward tries to reassure the guest by intercom while trying to find the malfunction. He detects the robot spiders in the ducts, and one of them enters his office and jumps onto his desk. It lowers the sun filter and sabotages the control. The Steward tries to close the filter without success as pure unfiltered sunlight blasts into his office.
The Doctor compares notes with Jabe and says he's not convinced by the Steward's broadcast. She offers to help him access a maintenance hatch, and then wonders what relation Rose is to him. Angry, Rose tells them to go and tries to start a conversation with Cassandra. The snobbish "pure" human talks about herself and believes herself the last true human and dismisses the rest as mongrels. She suggests Rose gets some plastic surgery and Rose angrily snaps that there's nothing of Cassandra left that's anything human. Rose storms out, but the Adherents of the Repeated Meme follow and knock her unconscious.
As they go to the computer access hatch, Jabe says that she's there to pay her respects as a descendent of Earth: Earth's tropical rainforests are her ancestors. She says that the computer is infallible but the Doctor isn't convinced, and notes that if it does malfunction they're on their own. As he tries to get the hatch open, she wonders why he's seeking out danger and wonders if it's because he has nothing to lose. She says she knows from her scan what race he is, and the Doctor is clearly upset at the mention of his people. He gets the hatch open and they find the central computer, accessible via a catwalk with giant blades swirling overhead. They find a robot spider and conclude that it's responsible for the sabotage... and someone placed the robot spiders on Platform One. Worse, there are only ten minutes until the Earth is destroyed and the expanding sun sweeps over their position. They go to the Steward's office and find him vaporised by the intense heat. As the Doctor tries to pinpoint the malfunction, he realises that another filter is programmed to open in a viewing gallery. Going there, he finds Rose locked inside. The filter begins to descend and Rose hugs the floor as the heat above her increases. The Doctor manages to stop the filter just in time but the door is damaged by the heat and he can't get it open. He goes after the person responsible, leaving Rose behind for the moment.
Jabe briefs the other guests on the situation and Cassandra accuses the Face of Boe of betraying them. The Doctor arrives and says he can expose the saboteur: he's programmed a robot spider to return to its owner. He sets it down and it scuttles briefly toward Cassandra, but returns eventually to... the Adherents of the Repeated Meme. However, the Doctor reveals that they're remote-controlled android. They gave out the disguised robot spiders as gifts to bypass the security protocols. The Doctor reactivates the robot spider and this time it continues to its true controller: Cassandra. She orders her surgeons to hold everyone at bay with acid-filled sprayers and explains that she planned to manufacture a hostage situation and sue the Platform One owners. Now that her original plan can't succeed, she's going with her secondary plan: escape and kill everyone on-board. She owns shares in all their companies and their value will triple upon their deaths. She'll use the money to ensure that she, not Rose, is the last true human in the universe. She detonates the robot spiders by remote control, destroying the computer and dropping the force fields. She teleports away with her attendants, leaving the others trapped.
The Doctor realises the computer must have a reset switch and he and Jabe return to the computer. The reset switch is on the computer, on the other side of the fans. As the heat has increased, the fans have accelerated to keep up. The override switch on the fans is broken and someone has to stay to hold it in place. Jabe volunteers and tells the Doctor to go. She informs him that she knows who he is: a Time Lord. The Doctor passes the first two sets of fans, but the rising heat cracks the viewing windows. Shafts of super-intense sunlight spear through the station, incinerating several of the guests and forcing Rose to shift her rapidly unsafe position.
The Doctor approaches the third set of fans but the increased heat causes Jabe to combust. She collapses, letting go of the switch. The fans speed up and the Doctor faces the swirling barrier, times things to the nanosecond... and steps between the blades. He activates the reset switch and orders the computer to reestablish the force fields. The expanding sun destroys the Earth and sweeps across the station... now protected once more. The fans return to their normal speed and as the Doctor returns the way he came, he looks sadly at Jabe, reduced to ash.
The Doctor and Rose return to the other guests and explain what happened. Realizing how Cassandra escaped, the Doctor breaks open Cassandra's ostrich egg gift. Inside is the teleportation feed she used to escape despite the intense heat outside. He reverses the feed and brings her back. Startled, she says that the chances of her being prosecuted are slim and could go on for years regardless. The Doctor notes that he didn't bring back her surgeons and their moisturizing sprays, and the station heat is still high. Cassandra's skin starts to crack and she begs for mercy. Even Rose is sympathetic to her agony but the Doctor refuses to help, saying that the end comes for all things and Cassandra's time has come. With one last scream, Cassandra ruptures, her blood and flesh spraying across the room.
The Doctor and Rose to go the gallery to watch what's left of Earth drift by, and they realise that everyone was so busy trying to stay alive that no one actually witnessed the destruction of Earth. They depart for Earth in Rose's time and look at the people walking down the streets, going on with their lives. Rose vows that unlike them, she won't take her planet for granted again. The Doctor, touched, explains that he is the last of the Time Lords and his people were wiped out in a great war. Now he travels alone... except for Rose. He asks if she still wishes to accompany him and she agrees, on one condition: they get some chips first.