The Doctor
The Doctor (who has many aliases and nicknames) is the central character in the long-running BBC television science-fiction series Doctor Who, and also features in a vast range of spin-off novels, audio dramas and comic strips connected to the series.
To date, ten actors have played the role in the television series (including the 1996 television film), with these changes being explained by his ability to regenerate. Several other actors have played the character on stage and film, in audio dramas, and in occasional special episodes of the series. David Tennant currently portrays the tenth incarnation of the Doctor.
The Doctor is a Time Lord, an extraterrestrial from the planet Gallifrey, who wanders time and space in an internally vast time machine called the TARDIS — Time And Relative Dimension(s) In Space.
The Doctor is an adventurer and scientist with a strong moral sense. He usually solves problems with his wits rather than with force, and is more likely to wield a sonic screwdriver than a gun; although he has been seen to use weapons as a last resort.
As a time traveller, the Doctor has been present at or directly involved in countless major historical events on the planet Earth and elsewhere — sometimes more than once. In the 2005 series premiere, "Rose", it is revealed that the Ninth Doctor was instrumental in preventing a family from boarding the Titanic prior to her fateful voyage. In "The End of the World", the Doctor claimed to have been on board and survived the Titanic's sinking to find himself 'clinging to an iceberg.'("It wasn't half cold") The Fourth Doctor also mentioned this event in Robot and The Invasion of Time, where he claims the sinking was not his fault. The final episode of the current season finishes with the prow of the Titanic bursting into the interior of the TARDIS.
By the time of his ninth incarnation, the Doctor believes himself to be the last surviving Time Lord following the Last Great Time War, although he learns in his tenth incarnation that the Master also survived ("Utopia"). Despite the Doctor's desperate attempts to save the Master from his evil ways, the Master is shot by his wife and refuses to regenerate, seemingly leaving the Doctor alone once more ("Last of the Time Lords"). However, the final scenes involving the Master's body leave the possibility of future regeneration open.