The Doctor and Donna have landed on the planet Midnight, which has a sun that emits Xtonic radiation. Donna prefers to get a (filtered) suntan at the luxury palace, while the Doctor decides to take a passenger-truck tour to see the Sapphire Falls. The Doctor boards the truck, the Crusader 50, and is greeted by the Hostess. The other passengers include Professor Winfold Hobbes and his assistant Dee Dee Blasco; Biff and Val cane and their teenage son Jethro; and businesswoman Sky Silvestry. The Hostess has them strap in then closes the window shields, protecting everyone against the Xtonic radiation which can vaporize a person in a matter of seconds.
The passenger truck departs and Driver Joe describes their route, saying they'll arrive at the Sapphire Falls in four hours. The Hostess turn on the various entertainment channels, annoying everyone. The Doctor short circuits the channels and the passengers chat among themselves. Professor Hobbes is familiar with Midnight, and notes that it is supposedly uninhabited but no one has been able to adequately explore because the radiation.
The truck stops and the Hostess claims that the pilot is stabilizing the engines. The Doctor cheerfully pops up front and quickly realizes that Driver Joe and his trainee, Mechanic Claude, are having engine problems. Joe assures him that a rescue team will be there in an hour. They open the shields and look out through the filtered windows, seeing an ancient city. Claude notices a shadowy figure moving toward the truck but they're forced to close the shields against the radiation before Claude can identify what he saw.
The Doctor goes back to reassure the passengers, who start to panic until Dee Dee, a mechanic, assures them they have plenty of oxygen. They're interrupted when something starts banging on the hull: Hobbes insists no one could possibly survive out there. The intruder moves to the door and starts duplicating the passengers' knocks when they respond. Sky is growing increasingly panicky and cowers in the seats near the door. The knocking approaches the door and then something dents the dense material. The lights go out and the Doctor tries to calm everyone down. Using hand torches, they find Sky, who is surrounded by torn-up seats. Sky seems to be in a state of shock and doesn't respond to them. The door and walls are intact, and the Hostess tries to contact Joe and Claude. She opens the door and realizes that the pilot's cabin has been ripped off the ship. She's able to seal the door in the six seconds before the emergency pressure seal gives away.
The Doctor insists they are safe but Sky then turns to face them and initially doesn't say anything. However, when the Doctor starts talking to her she mimics his words. When the other passengers start speaking, she mimics each of them as well, a bare second after they speak. The situation gets steadily worse as the possessed Sky refuses to explain itself to the Doctor, and the other passengers are increasingly frightened as it seemingly steals their voices. Further, it speaks simultaneously with them instead of a half second behind. The Doctor moves everyone away while Dee Dee wonders if some monster has possessed Sky. Jethro speculates that the intruder focused on the passenger who was the most frightened, and needed someone emotionally vulnerable to take over.
The Doctor believes that Sky can still be saved, but the other passenger insists that they throw "Sky" out before she takes over any of them. The Doctor refuses to let them kill one of their own, but the passengers turn against him, wondering where he came from and why he has an obviously fake name of "Dr. John Smith."
The possessed Sky suddenly focuses on the Doctor, ignoring everyone else and only impersonating the Doctor. He once again tries to communicate with the entity but now it's speaking his words before he can say them. He starts echoing them while Sky regains her free will. She says that the entity has traveled from her to the Doctor. Hobbes pulls her away but Dee Dee doesn't believe the entity has moved on. Hobbes derides her for believing she's smart enough to think she can figure things out. The paralyzed Doctor can only echo Sky's words, even though Dee Dee thinks the entity is draining his life force.
Biff, Hobbes, and Jethro finally have enough. At Sky's urging, they haul the Doctor to the pressure seal and prepare to throw him out. However, the Hostess realizes that Sky is still imitating what the Doctor said earlier and is taking his words from earlier in the trip. She grabs the possessed Sky and pulls her out through the pressure seal, sacrificing herself to seemingly destroy the entity as well. The Doctor is freed, as the other passengers consider what they've done. As the rescue team arrives, the Doctor and the others realize that no one ever learned what the Hostess' real name was.
Later at the luxury palace, the Doctor and Donna are reunited and he admits that he doesn't know if the Hostess destroyed the creature, or what it wanted. The luxury palace will set up somewhere and the entity or others like it will be left to their planet. However, as Donna repeats the Doctor's expression "Molto benehe," he asks her never to imitate him again.