With the help of TV Guide's Aubry D'Arminio, we've come up with five keys to this sci-fi world.
1. The Doctor is a Time Lord, a humanoid alien whose mission is to save civilization from those who want to destroy it, particularly the Daleks, cyborgs who have a fascist view of the world and want to destroy all other races.
2. The Doctor travels in a TARDIS, a time machine disguised as a "police box" — basically, a big phone booth used to contact police, which was a common sight in London when the show debuted in 1963.
3. He is usually accompanied by one to three "companions." He sometimes forces people to join him, but he usually persuades them to come by promising great adventures.
4. He has the ability to regenerate himself 12 times. While he's been played by different actors, it's always supposed to be the same character.
5. The war between the Daleks and the Doctor's allies became so brutal that at one point he killed off both sides. It was the only way he could see to save the universe.