I wouldn't say that. The way I see it, every life is a pile of good things and bad things. Hey. The good things don't always soften the bad things, but vice-versa, the bad things don't necessarily spoil the good things and make them unimportant. ~ The Eleventh Doctor
'The Doctor' is a centuried-old alien which travels through time and space in his 'TARDIS', frequently with human companions.
To date, twelve actors have headlined the series as the 'The Doctor' with the transition from actor to actor explained within the narrative of the show through the plot device of 'regeneration' a biological function of his race allowing them to completely change the celluar structure and appearance so as to recover from a fatal injury. Several other actors have played the character on stage and film, in audio dramas, and occasional special the series. The character's enduring popularity led the 'Daily Telegraph' to dub him 'Britains favourite alien'. The incumbent Doctor is the Twefth Doctor, played by Peter Capaldi, who took over the role of Matt Smith in the 2013 christmas special 'The Time Of The Doctor'.
As a Gallifreyan the Doctor looks like a human on the surface, but there are differences. One well known difference is that he has two hearts. As a Time Lord the Doctor can 'regenerate' if his body is badly hurt and about to die. During the regeneration the body is healed and re-newed which includes a different appearance and a changed personality. Because of this the Doctor can be played by by different actors. This made it possible for the series to be filmed for over 50 years without having to change the character.
Up until now 'The Doctor' has regenerated 12 times and each incarnation has been played by a different actor.
1. First Doctor - William Hartnell (1963-1966)
2. Second Doctor - Patrick Troughton (1966-1969)
3. Third Doctor - Jon Pertwee (1970-1974)
4. Fourth Doctor - Tom Baker (1974-1981)
5. Fifth Doctor - Peter Davison (1981-1984)
6. Sixth Doctor - Colin Baker (1984-1986)
7. Seventh Doctor - Sylvester McCoy (1987-1989 , 1996)
8. Eighth Doctor - Paul McGann (1996)
9. War Doctor - John Hurt (2013)
10. Ninth Doctor - Christopher Eccleston (2005)
11. Tenth Doctor - David Tennant (2005-2010)
12. Eleventh Doctor - Matt Smith (2010-2013)
13. Twelfth Doctor - Peter Capaldi (2013-Present)