The Doctor and Donna take a holiday on the crystalline planet Midnight, which orbits close enough to its sun that the Xtonic radiation exposure would vaporise any living thing walking unprotected on its surface. Donna opts to relax at a spa while the Doctor takes a four-hour shuttle bus ride to the Sapphire Waterfall. Other passengers include the Cane family — Val, Biff, and their teenage son Jethro — Professor Hobbes and his assistant Dee Dee Blasco, and businesswoman Sky Silvestry. The staff are the driver Joe, trainee mechanic Claude, and a steward who is only referred to as 'the Hostess'.
The trip initially goes smoothly despite the shuttle being rerouted to a new course, but suddenly the shuttle stops. The Doctor checks with the shuttle's driver and mechanic, confirming that there's nothing wrong with the vehicle. He convinces them to open the shutter to look outside, and the mechanic believes he sees a shadow moving towards the bus. The crew calls for a rescue vehicle while the Doctor returns to the main cabin.
A few moments later, something begins knocking on the shuttle's hull, copying the passengers when they knock back. The knocking moves around the shuttle, making its way towards Sky Silvestry, apparently the most frightened of the lot, and dents the door she is standing by. The lights then temporarily fail and the shuttle is violently rocked. When the lights are restored, the seats near Sky have been ripped off the floor and she is cowering in the corner. An attempt to speak to the cabin crew reveals that their cabin has also been ripped away, exposing Joe and Claude to the deadly sunlight.
Sky initially remains motionless, but is coaxed into turning around by the Doctor. Attempts to get her to speak only cause her to repeat what she is told, making it clear that Sky is no longer in control. The delay between Sky's repetitions becomes shorter, until eventually she begins speaking in exact unison with the passengers. Cabin fever sets in, and the passengers contemplate throwing her outside. The Doctor's attempts to calm the situation fail when the passengers become suspicious of him, especially when he is unwilling to reveal his name. This is only amplified when Sky focuses solely on repeating the Doctor's words.
As the Doctor tries to reason with Sky, she begins speaking his words first, and the Doctor quickly becomes the one doing the repeating. Sky is now able to move, while the Doctor is paralysed. Most of the passengers reason that whatever was in Sky has now passed into the Doctor, while the hostess and Dee Dee reason that this is just the next step: stealing the voice of another. The other passengers refuse to listen and begin to drag the Doctor towards the nearest door after being goaded by Sky. However, the hostess realises that Sky is not talking in her own voice when she uses two phrases the Doctor had used earlier. Before the other passengers can throw the Doctor out, she sacrifices herself by dragging Sky out of another door. The Doctor slowly recovers, and as the passengers wait for the rescue shuttle, he realises that no one knew the hostess's 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 do that.
Doctor David Tennant (Tenth Doctor)
Companion Catherine Tate (Donna Noble)
Billie Piper – Rose Tyler
Lesley Sharp – Sky Silvestry
Rakie Ayola – Hostess
David Troughton – Professor Hobbes
Ayesha Antoine – Dee Dee Blasco
Lindsey Coulson – Val Cane
Daniel Ryan – Biff Cane
Colin Morgan – Jethro Cane
Tony Bluto – Driver Joe
Duane Henry – Mechanic Claude
Writer Russell T. Davies
Director Alice Troughton
Script editor Helen Raynor
Producer Phil Collinson
Executive producer(s) Russell T. Davies
Production code 4.10
Series Series 4
Length 45 minutes
Originally broadcast 14 June 2008