‘Doctor Who’: What you need to know about the U.K. sci-fi series

A strange man in a box.

It’s a simple premise, but one that has sustained "Doctor Who" for decades. Since first airing in 1963, the endearing sci-fi series about a time-traveling stranger spanning the stars in his TARDIS, a spacecraft in the form of a blue British police box, has become a cultural institution, but one that had largely faded from view before its massively successful BBC relaunch in 2005.

Currently helmed by Steven Moffat ("Sherlock"), the new "Who" is more popular than ever, with a fresh generation of Whovians (the preferred term for Doctor devotees) fervently anticipating his every move. This Saturday the series celebrates its 50th anniversary with the global debut of "The Day of the Doctor," a special event that sees the return of David Tennant’s Tenth Doctor (among other familiar faces), and promises to be one of the most momentous episodes in the show’s history.

As the hour approaches, here are five things to know as "Doctor Who" turns 50.

There’s more of him

than we thoughtAs the last Time Lord and subject to frequent reincarnation, the Doctor has a (very) long and tangled timeline. Last season’s gripping finale hinted at a previously unknown War Doctor, played by the estimable John Hurt.

It’s a boy’s only

adventure, so far

The Doctor has worn many faces, but all of them white, male and British. With news that Smith was departing, it was rumored that perhaps a woman or a minority would take his place, before Scottish actor Peter Capaldi ("The Thick of It") was ultimately named the Twelfth Doctor.

He’s never aloneJoining the Doctor has been a series of human companions (also, one robot dog), whose mortality and vulnerability are often the Achilles’ heel of the alien traveler. Most recently the role has rested with Jenna Coleman as the mysterious Clara Oswald, who will continue into next season.

Here there be monsters

A hallmark of the show is the showdowns between the Doctor and various human-exterminating foes, from clunky classics like the Cybermen to fresh evils like the Weeping Angels. It’s a closely guarded secret where the Doctor is going next, but we already know the dreaded Daleks are due to return in "The Day of the Doctor"

Christmas is bittersweetThe annual "Doctor Who" Christmas special usually pairs a heartwarming holiday tale with a sci-fi twist, and is also occasion to bid a fond farewell to the old Doctor before a new face takes his place. Moffat has promised an unforgettable entrance for the Twelfth Doctor.