The episode continues from the end of the third series finale "Last of the Time Lords", when the TARDIS collided with the Titanic. To investigate further, the Doctor runs the TARDIS's self-repair programme and boards the Titanic, where he discovers that the "ship" is an interstellar cruiser from the planet Sto. Modelled after the Earth ocean liner of the same name, the ship is orbiting present-day Earth to observe "primitive cultures"–specifically, Christmas. He decides to stow away, only telling waitress Astrid Peth (Kylie Minogue). The Doctor joins Astrid on a brief excursion to Earth, along with married couple Morvin and Foon Van Hoff (Clive Rowe and Debbie Chazen), an alien with a spiked red head named Bannakaffalatta (Jimmy Vee), and historian and guide Mr Copper (Clive Swift). However, the populace of London fled in fear of a third consecutive extraterrestrial attack, and only a few people remain in the city, most notably a newspaper seller called Wilfred Mott (Bernard Cribbins).
After the party return from the excursion, the ship's captain, Hardaker (Geoffrey Palmer), dismisses all his officers to commit an act of sabotage; he magnetises the hull of the ship, causing meteors to collide with it. The resulting collision kills most of the passengers on-board and draws the ship to an extinction level collision with Earth. The Doctor makes contact with Midshipman Alonzo Frame (Russell Tovey), a crew member who survived the collision, to help him stabilise the ship.
En route to the bridge, the Doctor's party are repeatedly attacked by the Host, androids resembling angels who were programmed to kill survivors of the collision. The Doctor breaks from the party and attempts to reach the control point for the Host. He is taken to the Host's leader, former cruise line owner Max Capricorn (George Costigan). He was bitter about being forced out of his own company and plotted the Titanic's inevitable collision with Earth to bankrupt the company. To save the Doctor, Astrid rams Capricorn with a forklift. Astrid and Capricorn ride off a precipice to their deaths.
The Doctor uses the Host to reach the bridge, where he uses the heat from entry into the Earth's atmosphere to restart the ship's engines. After stabilising the ship, the Doctor realises that the teleport bracelet Astrid was wearing backed-up her molecular pattern. However, the damaged system can only partially regenerate Astrid. The Doctor reluctantly allows her to dissipate into atoms, so she can fulfil her dream of exploring the universe. The party then amicably part ways, vowing not to forget the Doctor's and Astrid's actions.
Doctor David Tennant (Tenth Doctor)
Companion Kylie Minogue (Astrid Peth)
George Costigan – Max Capricorn
Clive Swift – Mr Copper
Gray O'Brien – Rickston Slade
Russell Tovey – Midshipman Alonzo Frame
Debbie Chazen – Foon Van Hoff
Clive Rowe – Morvin Van Hoff
Jimmy Vee – Bannakaffalatta
Geoffrey Palmer – Captain Hardaker
Bernard Cribbins – Wilfred Mott
Nicholas Witchell – Himself
Paul Kasey – The Host
Jessica Martin – Voice of the Queen
Writer Russell T. Davies
Director James Strong
Script editor Brian Minchin
Producer Phil Collinson
Executive producer(s) Russell T Davies
Production code 4.X
Series Series 4
Length 72 minutes
Originally broadcast 25 December 2007