At the International Gallery in London, security guards watch over a rare jeweled cup. They establish a laser barrier and organize around it. They are unaware that a female thief is dropping in from the ceiling to grab the cup and then ascend. A guard turns to find the cat toy that she's replaced it with. The thief, Lady Christina de Souza flees through the museum as the alarms go off and the lights come up. She gets outside and discovers that the police, led by DI McMillan, have already captured her getaway driver. Christina gets to the streets but the police close in. She finally leaps onto a double-decker bus and offers the driver diamond earrings as bus fare. He accepts and they leave. As the bus departs, the Doctor boards the bus and sits down next to Christina and offers her a chocolate Easter egg.
The police spot Christina as the bus leaves and give pursuit. Meanwhile, the Doctor chats to Christina about Easter and then gets a signal on a device. He notes that they have excitation and notes that he's picking up something strange.
McMillan and his men pursue the bus into a tunnel and seal off the north end. Meanwhile, the Doctor explains that his rhondium detector is sensing something. Another passenger, Carmen, tells her husband Lou that she can hear voices but he doesn't know what she's talking about. The Doctor suddenly tells everyone to hold on tight. The bus shakes and Carmen yells that the voices are screaming. A white light flares through the bus.
The officers at the north end of the tunnel report that there's no sign of the bus. McMillan confirms that the bus disappeared from in front of him.
The Doctor and the others step out of the bus onto the surface of a desert planet with three suns. They're unaware that an insectoid creature is watching them on a monitor.
Lou tries to encourage Carmen to get out of the bus, but Carmen insists that the voices of the dead are crying out to her.
The driver confirms that the bus chassis is intact but that they're stuck in the desert.
McMillan has his men seal off the tunnel.
Christina introduces herself to the Doctor, who is busy tasting the sand. He notices something unusual but refuses to explain. The passengers wonder if he had something to do with what happened, and the Doctor explains that he was tracking a hole in the fabric of reality. It wasn't supposed to be dangerous, but it suddenly enlarged and they drove through it. The Doctor tosses some sand in to the invisible hole, triggering it at both ends. However, he explains that they can't go through. The bus driver runs into it but is burned to the bone by the time he passes through the other side.
McMillan, seeing the skeleton emerge, calls in UNIT.
The Doctor explains that the metal framework of the bus protected them from the energy discharge. Christina realizes that they simply have to figure out how to drive the bus back through the energy hole, and says they need to appoint a leader. She takes charge and orders everyone else back into the bus, including the Doctor. He obediently follows her orders and goes inside. Christina then has everyone introduce each other and tries to pull them together as a team. She tells the Doctor to explain, and he informs them that they were in the wrong place at the wrong time. Carmen disagrees, explaining that someone created the wormhole for a reason. Lou explains that his wife has been psychic since childhood. The Doctor confirms that she has a low-level psychic ability and asks her what is out there. She tells them that something shining is coming, riding on the wind: Death. The Doctor manages to calm them down, and then asks one of the passengers, Angela, where she was going. Angela says she was going home to her family and then gets the others to focus. When he asks Christina what she was doing, all she says is that she was going far away. The Doctor tells them that he'll get them all home.
UNIT moves in under the command of Capt. Erisa Magambo. She secures the area and orders McMillan back. He wants Christina but Magambo ignores him and orders her men to fire at the first sign of hostile activity.
The Doctor has the passengers place the duckboards down beneath the wheels to provide traction, and Christina produces a folding shovel from her backpack. Much to the Doctor's surprise, she then produces an axe. Angela tries to start the bus but the engine is clogged with sand. One of the teenage passengers, Barclay, worked in a garage and they have him start on the engine. The Doctor starts heading across the desert and Christina insists on following him. The insectoid creature watches them with interest.
As they cross the desert, the Doctor comments about how Christina has unusual tools and is afraid of sirens. She refuses to answer his questions and asks him who might have brought them there. He says that they need to get off the planet, and Christina introduces herself as a Lady. She realizes that the Doctor knows more than he's saying but he refuses to answer questions. They spot a storm cloud approaching but the Doctor isn't convinced it's a sandstorm. They run back to the bus and the Doctor modifies a cell phone and makes a call to UNIT.
On Earth, Magambo takes the call and the Doctor explains what has happened. She tells him that Dr. Malcolm Taylor, their scientific advisor, is on site, and puts him on the phone. The scientist is thrilled to hear the Doctor and tells him that he's picking up major energy readings from the wormhole and determined a way to measure the wormhole. The Doctor congratulates him and tells him to continue working, and then hangs up.
The Doctor and Christina go back out into the desert and take a photo of the storm with a cell phone so they can send it back to Malcolm for analysis. They notice that there's something metallic in the approaching storm. On the bus, Carmen warns that something fast and strong is riding the storm, something that devours.
Christina hears a chirruping noise and then a grunting. She turns and sees an alien creature, an armed Tritovore humanoid with the head of a fly, approaching them. The Doctor tries to speak to it and it orders them to move as it escorts them at gunpoint. It takes them to a wrecked ship and the Doctor realizes that its captor crashed there. The Tritovores inform the Doctor that he will suffer for his crimes and believe he is the one who caused them to crash. The Doctor tries to explain and the Tritovore believe him. He asks them to send out a probe to analyze the storm, but the ship has lost power. He manages to restore some of the systems and sends a probe out. The Doctor then confirms they are in the Scorpion Nebula on the planet San Helios. The Tritovores came there to negotiate the disposal of waste material. Christina realizes that the Doctor is an alien. The Doctor brings up an image of a Tritovore city but then explains it was taken a year ago, and now the planet has become a desert. He explains that something reduced everything and everyone to sand. Carmen is hearing everyone die.
Malcolm calls the Doctor to inform him that the wormhole is growing. Magambo wants to know if it's a danger, Before she can get an answer, the Doctor takes a call from Barclay and the other teenager, Nathan, who inform him that Angela inadvertently used up all the gas. They have no way to move the bus into the wormhole. The Doctor, shocked, lapses into silence until the Tritovores inform him that the probe has reached the storm... and it's not a storm. It's a swarm of billions of flying creatures. One of them eats the probe but not before the Doctor confirms the creatures will be there in 20 minutes. They use the wormholes to move from planet to planet, stripping each one bare and moving on to the next. The Doctor realizes that the creatures fly faster and faster, and their mass and acceleration rip a hole in space. The creatures have bones of metal, making them impervious to the wormhole's energies.
The passengers hear the approaching "storm," and Carmen talks about a girl. She says the girl will fly.
Christina points out to the Doctor that the Tritovore crashed for a reason and it had nothing to do with the wormhole. He asks the Tritovore crew, and they explain that their drive system failed. The Doctor sees the crystal nucleus of their engine and realizes he can use it to drive the bus. He goes down into the shaft to recover the crystal while Christina monitors him via comm units. Once he's gone, she rigs up the personal winch that she used at the museum robbery and drops down the shaft. The Doctor realizes too late what she's doing as she plummets down the shaft. The Doctor uses his sonic screwdriver and stops her right before she hits the security grid. He directs her to a red button and she turns it off, but she insists on continuing. As she goes down, the Doctor tells her of the places that he's been and discovers she has the cup hidden in her backpack. He asks her what she is doing with the cup and realizes that she stole it. The Doctor disapproves, until he's forced to admit that he stole the TARDIS.
A screeching noise echoes throughout the ship and the Doctor remembers that the ship crashed for some reason. He confirms that the Tritovores have an open-vent system and warns Christina that something must have flown into the ship. She spots the creature and reports that it's inactive. The Doctor notes that the ship's lower temperatures are inhibiting it, but her body temperature is catching its attention. Christina manages to grab the crystal and the clamp plate and the Doctor pulls her back up as the creature flies up after her. She activates the security grid behind her, stopping it, and gets to the top to turn over the crystal. The Doctor tells the Tritovore they need to return with him to the bus. They hear more noises and realizes that more of the creatures are caught within the ship. One slams through the hull, killing the two Tritovores. The Doctor and Christina are forced to flee.
As the Doctor and Christina run back to the bus, Malcolm tries to call the Doctor, who tells him to try later. They get to the bus and the Doctor tosses the crystal aside and places the clamps on the bus wheels. He then has Christine dial Malcolm on the cell phone and tells him to find a way to close the wormhole. Malcolm already has a way to do so. The Doctor then hooks up the clamp control and prepares to navigate the bus through the wormhole, while Magambo puts her men on alert.
The clamp mechanism doesn't work and the Doctor needs something metallic and malleable. She realizes that he wants the gold cup and she reluctantly gives it up. the Doctor uses it to make the connection.
On Earth, Malcolm completes his arrangements. Magambo tells him to close the wormhole immediately without waiting for the Doctor and the others to return. Malcolm refuses and Magambo draws her gun and repeats her order.
The Doctor tells everyone to hold on tight and starts the bus. The bus lifts off on the anti-gravity clamps and he flies it toward the wormhole as the swarm approaches.
Malcolm refuses to surrender.
The bus enters the wormhole and emerges on the Earth side of the wormhole seconds later as the UNIT soldiers look on in surprise. As the bus soars into the sky, the passengers look down on the city. The soldiers inform Magambo that the bus has returned safely. Creatures start to emerge from the wormhole and Magambo tells her people to open fire. The Doctor calls Malcolm and tells him to close the wormhole. When the scientist activates the circuits, the wiring bursts into flame. He warns the Doctor that it's not working, and the Doctor tells him to feed the signal back into the wormhole. When Malcolm does so, the wormhole closes just before any more of the creatures can come through. The UNIT soldiers bring down the few that made it to Earth. One of them heads for the bus and the Doctor swats it aside with the bus and the soldiers dispose of it. Christina kisses the Doctor while the other passengers applaud.
The Doctor lands the bus and the soldiers escort everyone off. They take Christina to the side while Malcolm embraces the Doctor. Magambo offers the Time Lord a salute, and he tells her that he'll use the TARDIS to nudge the wormholes to uninhabited planets. The Doctor then suggests she hire Nathan and Barclay as privates for UNIT. Magambo has the TARDIS brought in and leaves to fill in the paperwork on the incident.
As McMillan moves in on Christina, she runs to the Doctor and asks him to take her with him. He refuses, explaining that he's lost people who have traveled with him and he won't let that happen again. McMillan arrests her on suspicion of theft. As Carmen and Lou leave, Carmen gives the Doctor one more psychic warning: "he will knock four times."
As Christina is loaded into the police car, the Doctor covertly uses his sonic screwdriver to open her handcuffs. She slips out of the police car, gets into the bus, and flies away while the passengers wave to her. She stops above the Doctor and notes they would have been good together, and the Doctor notes that they were.