So, just who is Doctor Who, you ask? Well, the first thing to know is that the character is actually known as the Doctor (otherwise unnamed), as opposed to ever being called Doctor Who, which is simply the name of the long-running series. The Doctor comes from a planet called Gallifrey and those of his highly advanced species are known as Time Lords. The Doctor rejected the boring oligarchy of his people, however, and went rogue with his stolen TARDIS (more on this magnificent machine later). Despite looking the same, his people are biologically a bit different than human beings. They have two hearts, and among their most fascinating abilities is their power of regeneration. When a Time Lord dies (assuming the circumstances aren't too extreme), he or she is born again in a new body.
Doctor Who is a show that can seem pretty daunting to get into. After all, it's been running (with a break here and there) since 1963 with the same continuity. Lucky, the idea behind Doctor Who is very simple: the Doctor travels around space and time in a device called the TARDIS. He usually brings along a rotating selection of companions on adventures throughout the universe.
Time Lord regeneration is what has allowed for Doctor Who to continue on as series with the same protagonist for more than half a century. When the Doctor regenerates into a new body, it's not as simple as him or her looking differently. Each iteration of the Doctor has certain personality traits all their own. Here's the official lineup of Doctors!
The First Doctor - William Hartnell
The white-haired William Hartnell makes his debut as the Doctor in the very first episode, 1963's "An Unearthly Child". The title character of that episode is Susan, the Doctor's granddaughter, who attends school on Earth while the Doctor keeps the TARDIS in a nearby junkyard. When two of Susan's teachers wind up discovering the Doctor's secret, they wind up joining Susan as the Doctor's first companions.
The first Doctor tends to be a bit more curmudgeonly than his subsequent regenerations. By the time William Hartnell was done with the role, Susan had already been written out of the show in favor of new companions. Since Hartnell's death in 1975, the role of the first Doctor has been played by other actors, including Richard Hurndall and David Bradley. After all, when the Doctor travels in time, he sometimes runs into himself.
The Second Doctor - Patrick Troughton
Unfortunately, the actual first regeneration is lost to time. The BBC didn't do the greatest job with their archiving and, sadly, several Doctor Who episodes are lost, Patrick Troughton's 1966 debut among them.
A bit more laid back than the first Doctor, the second Doctor wears ill-fitting clothes and has a fondness for playing the recorder. When situations became dire, Patrick Troughton's Doctor is nevertheless every bit as clever as his predecessor.
The Third Doctor - Jon Pertwee
When Jon Pertwee took over as the third Doctor in 1970, Doctor Who changed a bit as a series. Now in color, the show follows a dramatic exit from Patrick Troughton, leading to the Doctor being judged by the Time Lords and then exiled to Earth. There, he works with UNIT (the fictional United Nations Intelligence Taskforce) to protect the Earth from extraterrestrial threats.
At one point called a "fancy pants" by his predecessor, Jon Pertwee's third Doctor comes off as a lot more debonair than either William Hartnell or Patrick Troughton's version. Eventually, the third Doctor regains the use of the TARDIS. UNIT's leader, Brigadier Lethbridge-Stewart, with whom the Doctor becomes quite close, remains a recurring character for some time and eventually the torch is passed to his daughter, Jemma Redgrave's Kate Stewart.
The Fourth Doctor - Tom Baker
The actor to play the role the longest, Tom Baker's reign as the fourth Doctor lasted from 1974 to 1981. He's arguably the best known Doctor, wrapped in a massive scarf underneath a mop of curly hair and often seen traveling with a robotic dog companion, K-9, and offering Jelly Babies to just about everyone in the universe.
The fourth Doctor actually traveled with another Time Lord, Romana, for a period. Initially played by Mary Tamm, Romana actually regenerates herself into a version played by Lalla Ward. Tom Baker made what is likely to be his last Doctor Who appearance in the recent 50th anniversary special, "The Day of the Doctor". There, he plays a mysterious man known only as The Curator.
The Fifth Doctor - Peter Davison
Peter Davison took over as the fifth Doctor in 1982. His clean cut look was a stark contrast to Tom Baker's performance and, during his three year run, Davison came up against a lot of classic Doctor Who foes.
The fifth Doctor also liked to wear a piece of celery on his lapel and displayed a love of the sport of cricket.
The Sixth Doctor - Colin Baker
No relation to Tom Baker, Colin Baker took over as the sixth Doctor in 1984 and himself played the role for three years. Clad in a coat of brilliantly clashing patchwork colors, the sixth Doctor was nevertheless incredibly determined and focused.
The sixth Doctor initially had trouble with his regeneration, causing him to almost murder one of his companions. There's a dark side to the Doctor that starts to become more apparent during the Colin Baker years.
The Seventh Doctor - Sylvester McCoy
The final Doctor of the show's original run, Sylvester McCoy played the part from 1987 until the show went off the air in 1989. While the show continued to explore the Doctor's dark side, McCoy also brought a lot of humor to the part.
While Tom Baker can claim the longest run of continuous episodes, Sylvester McCoy has a mighty Doctor Who reign from another perspective. The show would remain off the air until a TV movie in 1996. In it, McCoy reprises the role, regenerating as the story begins. While he may only have a dozen episodes to his name, he was apparently the Doctor for a lot of adventures we never saw.
The Eighth Doctor - Paul McGann
Paul McGann played the Doctor in a 1996 TV movie that was designed as a backdoor pilot for a new show that never came to be. While McGann's Victorian looking Doctor only headlined the one adventure, he would reprise the role in a short that tied to the 50th anniversary special.
The Ninth Doctor - Christopher Eccleston
Doctor Who was finally relaunched in 2005 with Christopher Eccleston as the ninth Doctor. A lot has changed for the character, though, with the Time Lords having been wiped out after a massive war. Now, the Doctor is the last of his kind.
Unfortunately, Christopher Eccleston did not enjoy his time on Doctor Who. Although it was always the plan to have him exit after one season, Eccleston elected not to return for the 50th anniversary special.
The Tenth Doctor - David Tennant
The appropriately named David Tennant became the tenth Doctor from 2005 to 2010, a period during which the show became immensely popular all over the world.
While the tenth Doctor is, for the most part, charming and optimistic, David Tennant would continue to explore a darker side of the Doctor, intensified by the things he saw and was forced to do during the war that killed his people.
The Eleventh Doctor - Matt Smith
In 2010, Matt Smith brought a childlike sense of wonder to his bow-tied eleventh Doctor. The bow tie itself was a callback to Patrick Troughton's second Doctor with Smith's years very much reflecting the Bob Dylan line, "I was so much older then, I'm younger than that now."
Matt Smith's eleventh Doctor defined youth as a state of mind, capturing the Doctor as an ancient spirit with a boyish smile.
The Twelfth Doctor - Peter Capaldi
Countering the youthfulness of Matt Smith's eleventh Doctor, Peter Capaldi -- a lifelong Doctor Who fan -- took over as the twelfth Doctor in 2013. Capaldi had previously guest starred as a different character during David Tennant's run and it is explained that the Doctor consciously borrowed that character's face.
The Thirteenth Doctor - Jodie Whittaker
It's likely the upcoming Christmas special where we'll meet Jodie Whittaker's thirteenth Doctor. It's the first female regeneration for the character, but it's been established that Time Lord gender can be pretty interchangeable.
The War Doctor - John Hurt
When Christopher Eccleston didn't return for the 50th anniversary special, a storyline was developed that revealed a secret regeneration known as the War Doctor, played by none other John Hurt. Although an official regeneration, the War Doctor doesn't really get counted like the others.
The Movie Doctor - Peter Cushing
Sort of the the Never Say Never Again of the Doctor Who franchise, 1965's Doctor Who and the Daleks was one of two feature films to star Peter Cushing as the Doctor. Neither it nor its 1966 sequel, Daleks -- Invasion Earth: 2150 A.D., tie into Doctor Who continuity in any way and you're probably better off just pretending they don't exist. Instead, lets take a trip inside the TARDIS!