The Titanic

In the final scene of "The Sound of Drums/Last of the Time Lords" we witness the Titanic breaching the walls of the TARDIS Console Room. As explained in the Children In Need special scene "Time Crash", this is because The Doctor left the TARDIS shields down.

Though momentarily stunned by this event, The Doctor is able to quickly repair the TARDIS walls and push the Titanic out. The TARDIS then materialises aboard the ship where The Doctor soon learns that the Titanic is in fact a large luxury spaceship from the planet Sto, orbiting present-day Earth. Intrigued The Doctor decides to mingle with the passengers, to enjoy the party, and so becomes a stowaway. The Doctor also meets waitress Astrid Peth, who reveals her own desire to travel the stars.

While socialising The Doctor gets the opportunity to go on one of the excursions that has been planed while the ship orbits the Earth. He decides to take Astrid with him and so they teleport to London along with couple Morvin and Foon Van Hoff, and a small alien called Bannakaffalatta. The ship's historian and guide Mr Copper gives the excursion party a bizarrely inaccurate explanation of human society, especially about Christmas, despite the fact that he claims to be an expert on the planet.

The TARDIS is Hit

Their trip is though short lived due some unexplained power surges aboard the Titanic. Unknown to The Doctor, and the passengers, the Captain commits an act of sabotage, causing meteors to collide with the ship. The Captain along with many members of the crew and passengers are killed when the meteors cause three major hull breaches, one of which sucks the TARDIS into space. The Doctor though is not too concerned about this as he knows that his time-machine will just land on Earth automatically.

With the teleport system offline, the engines losing power and the Titanic heading for an extinction-level collision with the Earth The Doctor leads a small group of survivors through the shattered vessel to reach the bridge so that they can regain control of the ship. But a number of complications are in there way – namely The Host - information androids resembling angels that have been reprogramed to kill everyone onboard.

The Doctor and Astrid

As the remaining survivors are picked off one-by-one The Doctor becomes more desperate and when Bannakaffalatta and both of the Van Hoffs are killed he makes a grim promise that "no more" will die. Bannakaffalatta's death though does give The Doctor their only effective weapon against the Host as Bannakaffalatta is actually a cyborg - something considered shameful in the society on Sto – and so The Doctor is able to use his power unit to produce an electromagnetic pulse to disable The Host in the vicinity.

Giving Astrid the power unit he instructs her to carry on towards the Bridge with the remaining survivors while he attempts to reach Deck 31, the place from which the Host seem to be controlled from. It is not long before The Doctor is trapped by The Host but by using a security protocol, he convinces The Host to take them to their leader. This turns out to be the cruise line's owner, Max Capricorn, who is hiding in an indestructible "impact chamber" on Deck 31. Max Capricorn is also revealed to be a cyborg, resembling a small wheeled vehicle. Having been forced out by the company's board of directors (due to their prejudice against cyborgs), he is seeking revenge. His plan for the Titanic to crash into a heavily-populated world will not only break the company, but see the board charged with murder, giving Max Capricorn his ultimate revenge.

Outnumbered by The Host and faced with death, The Doctor is saved by Astrid, who has managed to make a short-range teleport to his position. She rams Max Capricorn with a fork-lift truck, and in the struggle both are forced off a precipice and fall into the fiery engine of the ship.

Astrid Perth

With the Host no longer under Capricorn's control, The Doctor manages to finally reach the bridge and take control of the ship and so prevents it from colliding with Earth. With the danger over, The Doctor suddenly realises that there might be hope for Astrid after all as Mr Cooper confirms that a safety feature of the ship's teleport system is that in case of accident, it automatically holds in stasis the molecules of the affected passenger and as Astrid was still wearing a teleport bracelet at the time of her death, her pattern might still be stored in its buffers. However, despite desperate efforts, only a shadow of Astrid can be generated due to extensive damage to the teleport system. The Doctor watches her dissipate into motes of light that float free into space. This way, she can at least fulfil her dream of exploring the universe, forever.

With power restored to the teleport system The Doctor is able to return to Earth where he finds the TARDIS waiting for his return.