One of the main reasons why Doctor Who has built an enduring fandom over the past 50 years is this: you really don't need to watch much of it to figure out what's going on. Though the show has an incredibly rich mythology you can dive into if you want, it is designed so you can jump on board at just about any point.
If you want to start fresh with the 50th anniversary special, hitting screens in an unprecedented global simulcast Saturday, you can get away with it. That said, a little knowledge of the show's past and its unsolved mysteries will definitely enhance your experience.
See also: How the Internet Saved 'Doctor Who'
So if you're a complete beginner, if you're out of touch with recent developments, or even if you know every detail of the rebooted show (2005 onwards) but aren't so clear on the "classic" series (1963 to 1989), here are some key points that will help you sound smart at any viewing party.
Warning: if you're determined not to watch the anniversary special until you've seen all seven seasons of the rebooted show, then naturally the following contains spoilers.

The name of our hero is the Doctor. (Never call him Doctor Who, which is both the name of the show and a question posed within the show.) He is a Time Lord from the planet Gallifrey, who was exiled when he stole a time machine called the TARDIS, which got stuck in the shape of a British Police Box on a visit to Earth.
Any fan can tell you why, when, and what TARDIS stands for, but it doesn't really matter. Just remember that it's bigger on the inside than the outside, and can travel anywhere, any when — just not with a great deal of accuracy.
The Doctor looks human, has vowed to protect humans, and nearly always travels with one or two human companions. But he is subtly different from us. He's roughly a thousand years old, has two hearts, and like all Time Lords has the ability to "regenerate" into a new body when faced with a mortal wound. The number of times the Doctor has regenerated is currently in dispute, mostly because of ...

When we were reintroduced to the Doctor in 2005, we learned he was now the last of the Time Lords. Something called the Great Time War had taken place in the interim. Gallifrey had apparently gone to war with the Doctor's deadliest enemies, the Daleks: genocidal creatures in pepper pot-like metal shells. To end the Time War, the Doctor had somehow brought about the destruction of both the Daleks and the Time Lords.
That's about all we knew until earlier this year, when we learned about the existence of ...

The Doctor is usually committed to peaceful means. He almost never carries a weapon, preferring to fight using his wits and an all-purpose device called the Sonic Screwdriver. So how had he committed such a haunting act of mass slaughter?
We're starting to piece the answer together. In the final minutes of the latest season, we were introduced to a "forgotten" regeneration of the Doctor, (played by John Hurt), quickly denounced by the latest Doctor (played by Matt Smith). "What I did," replied Hurt enigmatically, "I did without choice."
And in a stunning five-minute mini-episode released last week, we saw the eighth incarnation of the Doctor (Paul McGann, who starred in the ill-fated 1996 TV movie) at the very end of his life. Having apparently tried to stay neutral in the Time War, McGann failed to save a crippled spaceship; the pilot chose to lock him in an airlock rather than let him save her life. "Who can tell the difference any more" between Time Lord and Dalek, she said.
Sustaining mortal wounds in the crash, McGann was revived and chose to transform into a "warrior" — a younger version of Hurt — because the "universe doesn't need a Doctor any more". This unusually focused regeneration came courtesy of ...

Newbies: here's a piece of supremely geeky Doctor Who knowledge you can use at the viewing party. The Sisterhood of Karn first showed up in a 1976 story called The Brain of Morbius. They were the keepers of the Elixir of Life, which was implied to have some ancient connection with the Time Lords' ability to regenerate.
Taking the elixir turned McGann's Doctor into Hurt's War Doctor. We still don't know if that counts as a regeneration, which matters because we've been told Time Lords can only regenerate 12 times. We also don't know how, or whether, Hurt regenerated into the 2005 post-Time War Doctor played by Christopher Eccleston.
What we do know is that Eccleston's Doctor soon regenerated into ...

Tennant's spry, dashing Doctor was the polar opposite of Eccleston's morose, haunted incarnation. For the first time the Doctor formed a romantic attachment to one of his companions, Rose Tyler. The Doctor and Rose were later separated by a dimensional anomaly; for reasons too complicated to relate briefly, she ended up with a human copy of the Doctor.
Sprinkled throughout Tennant's tenure were suggestions that the Doctor once had a romantic fling with Queen Elizabeth I, which may become relevant during the Anniversary Special. Shortly afterwards, Tennant's Doctor regenerated into ...

The latest and youngest incarnation of the Doctor, Smith pushed the character's alien eccentricity over the edge; he is the veritable "madman with a box." His latest companion, Clara, saved his life by jumping into a time vortex from the Doctor's future that contained his entire past. (Don't ask.) Suffice to say that Clara is now supposed to be lurking throughout the Doctor's many lives, just out of sight, saving him time and again, even inspiring the theft of the TARDIS in the first place.
And that should just about have you covered, except for ...

A one-time foe of the famous scarf-wearing fourth Doctor, played by Tom Baker, the Zygons look set to return for the Anniversary Special. (As is Baker himself, apparently.) We don't know that much about them, other than that they can change shape and have creepy whispering voices. They tried to conquer Earth by unleashing the Loch Ness monster, which was the kind of wonderfully cheesy thing monsters used to do in the classic Doctor Who series.
Oh, and one more thing:
That's the one thing showrunner Steven Moffat would like you to remember. At this point, the 50th Anniversary Special has been hyped to the point where even the greatest episode in Who history would have a hard time measuring up. So lower your expectations, sit back, have fun, don't expect it all to make sense, and get ready for a wild ride with the madman — well, several mad men — and their magical blue box.
Whovians: is there anything else newbies need to know before taking the plunge with the Anniversary Special? Let them know in the comments.
Image: BBC America