For her first trip in the TARDIS, the Doctor lets Rose choose their destination. She suggests a hundred years into the future and he does so but finds it boring. Rose suggests he do better and he first takes them to 12,005 and then even further into the future. They step outside and find themselves in a viewing gallery with the shades drawn. The Doctor opens them to reveal they're in a space station orbiting Earth. The sun starts to expand and the Doctor explains he's brought her five billion years into the future to watch the Earth die.
The Doctor and Rose go to the main reception area as a computer announce that some of the most wealthy elite people of the galaxy have arrived on Platform Earth to witness Earth's destruction. The National Trust has preserved the planet but has run out of money. They are finally letting nature take its course and people have gathered to watch Earth's destruction. As they go, they meet with Steward who doesn't recognise them. Using a sheet of psychic paper, the Doctor convinces him they have an invitation. They meet with the other guests, and Rose is shocked to meet aliens for her first time. Among the aliens are the hooded Adherents of the Repeated Meme, and Jabe, a humanoid tree from the Forests of Cheem. The sponsor is a giant head in a jar, the Face of Boe. Everyone exchanges gifts and the Adherent give metal spheres to everyone, including the Steward. The Doctor, unprepared but never at a loss, offers Jabe the gift of air from his lungs and she seems romantically taken with him.
The final guest to arrive is the last human, Lady Cassandra O'Brien. She's had so much plastic surgery that she's nothing but a face on a sheet of skin, attached to a brain in a jar. Her attendants constantly moisturise her to keep her from drying out. Cassandra offers gifts from her home planet: an ostrich egg which she thinks belongs to a dragon, and a 1950s jukebox that she mistakes for an iPod. Confronted with items from her own time, a shocked Rose leaves and the Doctor goes after her. As he leaves, Jabe scans him and is surprised at the readings. Meanwhile one of the spheres handed out by the Adherents, unseen, expands into a robot spider and scurries away.
Rose goes to a side room and watches the sun, and a blue-skinned maintenance worker arrives. Rose tries to talk to her and the woman, Raffalo, finally explains she needs permission to speak. Rose agrees and Raffalo goes to work opening a maintenance hatch while explaining that she's from Crespallion. As she talks about her people, Rose realises how divorced she is for her time and place and leaves, depressed. Raffalo enters a service duct and finds one of the robot spiders. She thinks it's a new maintenance robot but other robot spiders arrive and drag her into the ductwork.
The Steward is in his office when the computer notifies him that they've found an unidentified blue box. The Steward makes a public announcement that teleportation isn't allowed and has his attendant remove the TARDIS while giving the Doctor a claim ticket. Meanwhile, the robot spiders are spreading rapidly throughout Platform One. One of them finds Rose and examines her, but quickly ducks away when the Doctor arrives. She admits she's suffering from culture shock and asks where the Doctor is from, but he evades the question. Instead he explains why all the aliens are seemingly speaking English. The TARDIS is altering her perceptions, and she's not too thrilled with having her brain tampered with. She returns to the question of who the Doctor is. Angry at being pigeonholed, he refuses to answer. She tries to call home but gets no signal. The Doctor, somewhat mollified and faced with a technical problem, adjusts it and Rose is able to call Jackie in the 21st century. They chat briefly but Rose is further depressed when she realises her mother died five billion years ago. She hangs up but Platform One shudders and the Doctor realises something is wrong.
The Steward tries to reassure everyone that it's merely a gravity pocket, but checks the infrastructure scanners for the serious malfunction it has to be. He detects the robot spiders in the service tunnels, and one of them enters his office. It lowers the sun filter on the viewing window and the 4,000 degree heat of the sun fills the office.
The Doctor insists something is wrong and talks to Jabe, who offers to let him use the maintenance hatch near her suite. She's curious what relation Rose is to the Doctor, irritating Rose. The Doctor and Jabe leaves and Rose tries to talk to Cassandra. However, the last "pure" human dismisses all other humans as mongrels and has nothing in common with Rose. When Cassandra suggests that Rose get some plastic surgery of her own, Rose stalks off into the corridor... and the Adherents of the Repeated Meme knock her out.
As they go to the maintenance hatch, Jabe explains that she's descended from Earth's tropical rain forest and she's there in part to pay her respects. She claims that the Platform One computer is infallible but the Doctor isn't convinced. As he tries to open the hatch, Jabe questions his motives for seeking out trouble. She explains that she knows who he really is, and how much he's lost. The Doctor is touched, then gets the door open. It leads to the computer complex, with gigantic cooling blades sweeping across the maintenance catwalk leading to the controls. A robot spider emerges and Jabe catches it. Examining it, they figure it's responsible for the malfunctions... and there's only ten minutes until the nearby Earth is destroyed.
As the guests mingle, the Doctor and Jabe go to visit the Steward, only to find him vaporised. Examining the filter machinery, the Doctor realises it's been programmed to open another filter in a viewing gallery. The Doctor goes there and finds the door locked... and Rose inside. The filter descends from the ceiling down and rose hugs the floor while the Doctor attempts to override the programming. He succeeds but the heat has damaged the door lock. He has no choice but to leave Rose while he goes after the individual responsible for the sabotage.
Jabe is informing the other guests of what has happened, and Cassandra accuses the Face of Boe of luring them there. The Doctor arrives and reveals he's reprogrammed the captured spider to return to its programmer. He puts it on the floor and it eventually goes to the Adherents of the Repeated Meme. They gave the spheres out as gifts to get past the security systems. However, the Doctor removes their hoods to reveal they're androids, controlled by remote. He reactivates the spider and this time it goes to Cassandra.
Cassandra has her attendants threaten to use their sprayers, which are filled with acid. She admits she planned to create a hostage situation and then sue the Platform One owners to make a fortune. Now that she's been exposed, she's going to depart and leave the others to die. She owns stock in all of their companies and her shares will triple in value upon their deaths. She can then us the money to finance endless plastic surgery to remain the last pure human. She detonates the spiders by remote, destroying computer control and shutting off the station's force fields. She and her attendants teleport out, leaving the others stuck with no means of escape.
With time running out, the Doctor takes Jabe with him back to the computer core. They need to access the manual reset switch for the computers... which is on the other side of the catwalk and the fans. The fans speed up as the heat increases, and the lever stopping them won't stay in place. Jabe insists on staying to hold the lever despite the heat, and tells the Doctor to get going: he's a Time lord. The Doctor moves through the first two sets of slowly rotating blades, but the heat continues to rise. The viewing panels throughout Platform One start to crack, and shafts of sunlight slice across the viewing gallery where Rose is still trapped. The heat in computer core increases and Jabe bursts into flame. The fans speed up and the Doctor faces the third and final set of blades. With only ten seconds left until the sun expands, the Doctor closes his eyes, braces himself... and with perfect timing steps through the third set of swirling blade. He hits the reset switch and reactivates the computer, and orders it to re-raise the force fields. The sun's corona sweeps across Earth, destroying it, and then passes harmlessly pass the station. The fans return to their normal safe speed and the Doctor can only stare at Jabe's incinerated body.
Rose gets out of the gallery and returns to the other guests. The doctor meets her there and tells Jabe's companions of her valiant efforts. He then smashes open the ostrich egg to reveal the teleportation feed inside that Cassandra used to escape. He reverses the feed and brings her back. She defies them all, insisting she can keep the trial going for years. However, the Doctor notes that her attendants aren't present and the increased heat is causing her to dry out. Even Rose is taken aback but the Doctor coldly ignores Cassandra's pleas for mercy. He tells her that everything dies eventually, and her time has come. Everyone looks on as Cassandra's dried-out skin ruptures and explodes.
As the guests depart, the Doctor and Rose watch Earth's remnants drift outside. Everyone was too busy to watch it destruction. The Doctor takes Rose back to the TARDIS and they return to her time and place. As they walk down the street, Rose realises that having seen the end of the Earth, she won't take her planet and people for granted again. The Doctor admits that he is the last of his people, and his planet was wiped out in a war. Now he's alone but has someone new to share his experiences with. Rose agrees to stay with him but first wants to get some chips and they go to find them... together.