But the true genius of the show is the constant presence of a viewer surrogate: the companion. The Doctor almost always has at least one inexperienced time-traveler (usually human, usually a woman) aboard to ask him what planet they're on, what that button is for, why is that stuntman wrapped in green spray-painted bubble wrap, and how in the world am I supposed to pretend to be scared of him?

The Doctor also is Torn Between Two Worlds. He's an alien who's always hanging around humans and deeply cares about them, but will always be distant because of his Otherness. "I suppose, in the end, they break my heart," David Tennant's Tenth Doctor says. And yet he always comes back to them. Desperately groping for a connection—for family.

It's this heart-wrenching blend of intimacy and distance, of possibility and inevitability that make the show so uniquely engaging. The constantly changing actors playing the same character, along with a rotating cast of companions, is also unique in pop culture, and engenders an unrivaled level of fan passion and investment in the characters. Everybody has a favorite Doctor and a favorite companion, and everybody has suggestions for who the next Doctor should be and what kinds of adventures he—or she (it's really about time)—should have.

With former showrunner Russell T. Davies's relaunch of the series, he put an even finer point on the Doctor's loneliness: The Doctor is now the only Time Lord left, having destroyed both his own people and their mortal enemies the Daleks in the Last Great Time War. So, now he's Superman, too, only with more guilt and PTSD.

Davies's run has its critics, featuring as it did more than a handful of clunker episodes and melodramatic finales that leaned too heavily on people gaining godlike powers in the third act. But he managed to give both the Doctor and his companions satisfying character arcs, and the inevitable lachrymose farewells nearly always felt earned.

When Steven Moffat took over in 2010, introducing a new regeneration of the Doctor, played by Matt Smith, a lot of fans were thrilled. Moffat's episodes during Davies's tenure are consensus top-10 picks for the best episodes of the Davies years, if not ever.

Now, though, in the third full season with Moffat at the helm, what some viewers saw as troubling tendencies from those early episodes have multiplied. Television Without Pity recapper Jacob Clifton went so far as to say that Davies's Who is "a different TV show that shares basically a title and a prop" with the Moffat incarnation. Indeed, Moffat-vs.-RTD is the biggest Whovians-yelling-at-one-another-on-the-Internet fault line. (New Who vs. Classic Who debates tend to be courtly by comparison.)

Like any writer, Moffat has his pet themes and plot devices. His favorites mostly have to do with perception, memory, and the power of story. The Weeping Angels can only move when you're not looking at them. You can only remember the Silence while you're looking at them. Companion Amy Pond rebooted the entire universe simply by remembering it. His tendency to recycle ideas has prompted more than a few parodies; Moffat Bingo is my favorite.