Midnight

While visiting the crystalline resort planet of Midnight, the exposed surface of which is bathed in xtonic radiation due to its close orbit around its sun, The Doctor is unable to persuade the spa-going Donna from joining him on a long shuttle bus trip to see the Sapphire Waterfall. Alone, The Doctor joins the other passengers on the shuttle—Professor Hobbes and his assistant Dee Dee, the Kane family of Biff, Val, and Jethro, and businesswoman Sky Silvestry—as it sets out; when the hostess attempts to start the cacophony of en route entertainment, The Doctor disables the system, and the rest of the passengers readily join him in casual conversation.
The bus stops mid-route, the pilots citing a problem with its micropetrol engines and sending a request for help. The Doctor insists they raise the radiation shields briefly to see if there's anything outside that caused the problem, but they see nothing; however, as they close the shields, one pilot claims to have spotted something moving outside. Soon, a rhythmic knocking starts on the hull of the shuttle, mimicking the patterns that the passengers make on the walls. The knocking moves around the hull to where Sky is cowering; the shuttle is rocked briefly, causing the lights to fail, but when they return, they find that the pilot cabin has been torn away, killing the pilots, and that Sky appears to be possessed amid an array of damaged seats.The Doctor talks to Sky, but she can only repeat the words he or the other passengers say. However, as he talks to her more, the delay in her repetition decreases, and soon is repeating whatever anyone says simultaneously. The other passengers fear for what has possessed Sky, and cabin fever sets in; they start to accuse The Doctor, distrusting him for not revealing who he truly is. The Doctor realises Sky is now repeating only his words, and turns back to Sky to try to help her. Soon, it is the Doctor that is now repeating Sky's words, causing the other passengers to think the possession has moved onto the Doctor. Though Sky tries to encourage the other passengers to throw The Doctor out of one of the airlocks, both the hostess and Dee Dee, and later, Jethro, believe this to be the next stage of Sky's possession. When Sky uses archaic phrases that The Doctor had used at the start of the trip, the hostess realises Sky is still possessed and sacrifices herself to drag Sky out of the other airlock. With Sky gone, the Doctor regains his normal self while the other passengers come to grips with what happens. As they wait out for rescue, The Doctor realises that no one knew the hostess's real name. At the spa, a mournful Doctor reunites with Donna. When she tries to imitate one of the Doctor's phrases, he quickly tells her not to.