On the crystalline planet of Midnight, Donna is relaxing and sunbathing at a leisure palace. The Doctor calls and tries to convince her to come with him on a sightseeing tour on a space truck across the planet. She’d rather work on her tan, although the Doctor warns her that the Xtonic radiation is strong enough to vaporize anyone walking unprotected on the surface. Donna advises him to be careful but the Doctor asserts that nothing can possibly go wrong.
The Doctor boards and is given his complimentary passenger gifts by the Hostess. Professor Winfold Hobbes introduces himself to the Doctor along with his assistant Dee Dee Blasco. Looking around, the Doctor notices married couple Biff and Val Cane and their teenage son Jethro, and businessman Sky Silvestry. The Hostess greets them and they prepare for departure, closing the window hatches and warning there is no atmosphere on Midnight. Driver Joe gives them a description of their route and the fact they will arrive in four hours. The Hostess turns on the entertainment channels but the Doctor covertly shuts it down with his sonic screwdriver. He suggests they talk to each other and he’s soon making friends with everyone except Jethro. Sky notes her wife recently broke up with her.
Professor Hobbes lectures about how no one has ever fully explored Midnight due to the radiation. The truck stops and the Hostess directs them to their seats while Hobbes notes the truck never stops. The Hostess claims Driver Joe needs to stabilize the engines but the Doctor goes forward to talk to Joe and Mechanic Claude. They try lying but the Doctor realizes something is wrong and they say they’ll be picked up in an hour. The Doctor suggests they open the shields and they look outside, seeing a vast crystalline city. Claude notices what he dismisses as a shadow, and they close the shields when they begin to overheat. Claude sees the shadow again but can’t identify it, but thinks it was moving toward the truck.
The Doctor goes back to the passenger cabin and reassures the others, but Dee Dee realizes that the truck engines don’t need stabilization. The passengers begin to panic and the Doctor yells for quiet. He has Dee Dee explain that they have ten years’ worth of oxygen. However, something starts banging on the hull. Hobbes insists it is impossible for anything to survive outside but the knocking continues.
The new arrival moves to the door and when Biff Cane knocks on the door three times, the creature knocks back with the same pattern. Sky starts to panic in earnest and backs up the door… and something dents it. The power flickers out as the truck is shaken back and forth, and the Doctor tries to check on the others. He’s unaware that Rose’s face is on the monitor screen behind him. They use hand torches and see that the seats around Sky have been ripped out of the floor. The walls are intact but the Hostess is unable to contact Joe and Claude. The Hostess opens the forward door only to discover that the pilot’s cabin has been ripped off. They close the doors before Xtonic radiation can flood the interior. The Doctor assures them that they’ll be safe when the rescue team arrives but Jethro notices that Sky is still facing away from them and not responding.
The pounding has stopped but Jethro wonders if the creature is inside, and it was heading for Sky. Sky finally turns around but peers at them curiously and without recognition. When the Doctor speaks to her, she mimics his words and then the words of the other passengers. The Doctor gets everyone to quiet down and tries to communicate with “Sky” without success. The passengers begin to panic again, with “Sky” repeating everything they’re saying. The backup lights finally come up but “Sky” starts talking simultaneously with the passengers, repeating everything they say as they say it. The Doctor tries to stop her without success and then has the passengers move to the back, away from “Sky.” Dee Dee recites a line of poetry from “Goblin Market” about goblin men and “Sky” recites along with her. Jethro notes that Sky was the most frightened of them, and that’s what the invader needed.
The Doctor insists on trying to help Sky, trapped in her own body. The Hostess suggests they throw “Sky” out. Dee Dee explains that they can get the pressure seal open but the Doctor refuses to hear the suggestion. The passengers all decide to throw Sky out before she kills the rest of them. The Doctor promises to contain it once they get back to base but the passengers start to turn against him. The Hostess notes that the Doctor just turned up out of the blue, and seems gleeful about the entire thing. Biff suggests they throw the Doctor out as well. Professor Hobbes asks him what his name is but the Doctor can only give his alias of “John Smith,” and the passengers don’t believe him.
“Sky” suddenly stops imitating everyone… except the Doctor. He tries to find out what she wants but she starts speaking first, the Doctor echoing her words as he copies her. Sky says that the creature has let her go and moved to the Doctor. She tells the passengers she’s back to normal while the Doctor is paralyzed.
She asks the Professor to get her away from the Doctor and he pulls her away. Dee Dee is skeptical but Sky says the creature is inside the Doctor. The Hostess insists that Dee Dee explains, and she notes that the creature gets into sync and then moves ahead, stealing its victim’s voice. Hobbes orders her to shut up and berates her for being average. Biff grabs the Doctor and drags him toward the hatch, with Hobbes helping and Sky encouraging them. However, the Hostess realizes that “Sky” is still repeating the Doctor’s words, and has stolen his voice. She grabs “Sky,” hits the pressure seal, and sacrifices herself to kill the creature. The Doctor is released from his paralysis as the other passengers realize what they almost did. Twenty minutes later the rescue ship arrives and everyone realizes that no one knew the Hostess’ name.
Back at the leisure palace, Donna meets the Doctor and he admits he has no idea what the creature is, or whether it’s still out there. However, as Donna repeats something he says, the Doctor asks her not to do it: ever.