If Matt Smith is the only Doctor Who you've ever known, then you've seen some strange faces this season. In the first episode of this season, when the eleventh Doctor was facing down an alien set to destroy the Earth, it looked into his mind and saw several previous incarnations of the Doctor and various enemies he has bested.

Then again in episode 11, the Doctor had to transfer some of his memory to his roommate (yes, roommate), Craig Jones, and again, when saw some of the previous Doctors. If you saw 'Doctor Who: The Ultimate Guide,' you were brought up to date on the series reboot that started in 2005 with the Ninth Doctor, played by Christopher Eccleston, and continued with the Tenth Doctor, played by the immensely popular David Tennant.

But the history of this series stretches back to 1963, a year before the Beatles came to America, more than a decade before 'Star Wars' hit theaters, and decades before there was a BBC America.

So how to catch up? There are plenty of resources you can use to study the nearly five decades of 'Doctor Who' history. The BBC has several sites, and The Onion's AV Club has a great primer. Much of the series is on DVD, if you have time to start at the beginning (see our run down on the latest additions).

And if you do study the series beginning to end, you'll no doubt find a number of contradictions and continuity problems. Eleven different Doctors, two different TV series, a TV movie, and a variety of writers, producers, and BBC executives will do that. At his core, though, the Doctor is a refugee from his own race, the Time Lords, moving through time in his TARDIS, picking up humans to witness and participate in the spectacle of all space and time. He can also regenerate, but he comes back differently each time. He has yet to come back ginger, which is a recent wish.

The scope of 'Doctor Who' is limitless, and it would require volumes to cover in detail. But here is our own brief history of the show, with a rundown of the different Doctors.

The Doctors

The First Doctor
William Hartnell 1963-1966
'Doctor Who' began as a family series in 1963, with William Hatnell as the First Doctor, running from the rest of his race in a stolen time machine called TARDIS. He was accompanied by his granddaughter Susan and two school teachers, Barbara Wright and Ian Chesterton, hammering home the educational aspect of the show. You could hardly find an earlier place to begin - the first story found the Doctor and his companions helping cavemen discover fire.

With Hartnell, you can see how the writers and producers were trying to figure out what the character should be. He softened a bit from his original cranky demeanor, and with his white hair and stern gaze, came off as professorial.

The Second Doctor
Patrick Troughton 1966-1969
Troughton's Doctor is often referred to as a "cosmic hobo." And he looked a tad like Moe from the Three Stooges. Much more a comic figure, but no less cunning than Hartwell. His bumbling approach is sometimes compared to Matt Smith's take on the character. First companions are Ben and Polly, holdovers from the last few Hartnell episodes, and are replaced by Jamie and Zoe.

The Third Doctor
Jon Pertwee 1970-1974
Pertwee had the benefit of being the Doctor when the series went to color, a fact emphasized by his red jacket and blue lounge singer shirt. Pertwee was the picture of a refined but colorful Englishman. He was also linked to UNIT, the official organization for protecting the Earth from aliens (UNIT would come up again in the modern series, and also in the spinoff, 'Torchwood,' which was the unofficial protector of the Earth from alien technology). He also battles Silurians, the first inhabitants of the Earth. The Silurians came back into play this past season, as well.

The Fourth Doctor
Tom Baker 1974-1981
Before the reboot, Baker was very much the face of the series, with his wild, curly hair, and the most famous English scarf until Harry Potter took that title. The classic 'Doctor Who' series was never more popular than when Baker was in the role. He had the longest run as the Doctor, logging seven seasons. If you saw 'Doctor Who' as a kid in the States, chances are it was Baker you saw most frequently.

The Fifth Doctor
Peter Davison 1982-1984
Despite the celery sprig on his lapel, Davison's Doctor was supposed to be a bit more serious than his predecessor. Davison was already a known quantity from 'All Creatures Great and Small,' and also had the benefit of the increased popularity of 'Doctor Who' in America. Still, there is a decline in ratings as the show goes on, and Davison spent only three seasons in the role.

The Sixth Doctor
Colin Baker 1984-1986
Anyone who remembers Colin Baker's Doctor probably remembers the tacky, Wonka-like costume. The writing was a mess, and there was squabbling behind the scenes at the BBC. Despite a lot of familiar faces - battles with the Daleks and the Cybermen and a cameo by Second Doctor Patrick Troughton, the series was in trouble and Baker was ultimately fired after three seasons.

The Seventh Doctor
Sylvester McCoy 1987-1989
Yes, he bears an uncanny resemblance to Ted Buckland, the hapless hospital lawyer from 'Scrubs.' But that was part of the 'Doctor Who' tradition of setting up minimal expectations for the Doctor, who would always outsmart his enemies. There were a lot of great ideas in this season (see 'The Happiness Patrol,' where the Doctor visits a planet where sadness has been outlawed). But despite some inventiveness in design, the budgets were low, and it showed. 'Doctor Who' was canceled after season twenty-six.

The Eighth Doctor
Paul McGann 1996
McGann played the Doctor for the 90-minute, made for TV movie, pitted against Eric Roberts as The Master. It would have been a reboot for American audiences. It didn't work.

The Ninth Doctor
Christopher Eccleston 2005
Eccleston brought a violent intensity to the new 'Doctor Who.' The show retained its campiness, but with a much more sleek and modern look, taking advantage of CGI technology. The new Who looked more like an action hero than previous Doctors, and the new show was a hit.

The Tenth Doctor
David Tennant 2005-2010
Tennant is arguable the most popular of any of the Doctors. He was no less intense than Eccleston, but with a winning combination of daffy and hip. There is a definite sense of wonder and adventure in Tennant's tenure, and the feeling that there was nothing he couldn't handle with a couple of minutes to think and the sonic screwdriver at hand.

He faced the Daleks and the Cybermen (at one, point, while they were at war with each other), The Master, the Weeping Angels, and even his own race, when the Time Lords managed to find him. The popular spinoff 'Torchwood' also began in this era, which has now moved on from the BBC to Starz in America, with Russell T. Davies at the helm. Tennant's departure was also a bit out of the ordinary, as his Doctor struggled with the idea of regenerating as he saw it approaching.

The Eleventh Doctor
Matt Smith 2010-?
Smith is the image of a bow-tie wearing nerd, but a mad, brilliant, and fearless one. He kept Tennant's sense of wonder and added a bit of Troughton's clumsiness. Smith and new writer/producer Steven Moffat struck the perfect tone to follow up such a popular iteration of the series.