The midpoint of “Game of Thrones’” sixth season -- the season without those magnificent books as a guide -- has passed. What have we learned so far?

We’ve learned Hodor means “Hold the Door.” We’ve learned that dead isn’t forever if your name is Jon Snow (Kit Harington). We’ve learned that Arya (Maisie Williams) can see, and Sansa (Sophie Turner) can lead, that Theon (Alfie Allen) can stand down, that his sister Yara (Gemma Whelan) can stand up.

We’ve learned that Daenerys (Emilia Clarke) can still walk through fire, that Jorah (Iain Glen) loves her forever, that Tyrion (Peter Dinklage) can rule, that Ramsay (Iwan Rheon) is a purer form of evil than we even suspected. We’ve learned that Cersei (Lena Headey) has a new friend, and the High Sparrow (Jonathan Pryce) has new enemies.

As viewers and fans, we’ve learned a lot but to an extent, we’ve learned what we mostly expected to, doors aside. Some of these twists are less “reveals” and more “confirmation.” No one really expected Jon Snow to stay dead forever, no one expected Ramsay to seek sainthood either. In all its manifest details, the sixth season so far is really just a logical extension of the fifth. Character has remained true to character, while tone and direction have remained true to the gorgeous template George R.R. Martin established in “A Song of Ice and Fire.”

Then, that all changed Sunday night with “The Door.” The best episode of the sixth was the true game changer, the one episode that shifted an entire season, maybe an entire series, into a higher gear or onto a higher plane. The rules have changed, and so has the future. Whatever happens henceforth will be appended with an asterisk, essentially meaning “subject to change” or “proceed with caution.”

We really don’t have any idea what’s going to happen next week or the week after. Hold the door or damn the torpedoes -- take your choice -- but “Game of Thrones” is genuinely interesting once again, also genuinely compelling. We can talk about the future, about the endgame, but “The Door” established that it’s just talk, or clickbait, or speculation that will be vaporized or shredded as quickly as Bloodraven was Sunday. We really don’t know what’s going to happen next. That’s as it should be.

advertisement | advertise on newsday

“The Door” also achieved something else that has eluded “Game of Thrones” in this -- the bookless -- sixth season: A sense of deep tragedy, of the inevitability of fate. “Hold the Door” becomes “Hodor:” The mindless eternal repetition, the utter collapse of a mind reduced to the only three words he would hear moments before his death.

It’s downright magnificent, also downright chilling -- one of those twists that seem to offer new meaning to the term “existential crisis.”

The quick recap: “The Door” was Hodor’s (Kristian Nairn) origin story but the origin story no one saw coming. His endless repetitive refrain -- Hodor, Hodor, Hodor -- ultimately came to stand for his name, and essentially who he is or ever was, and also how he died. We learned this season that he once had a name (Wylis) through those flashback tours Bran (Isaac Hempstead Wright) took with the Bloodraven/ Three-eyed Raven (Max Von Sydow).

Yet Sunday -- in a time loop paradox that continues to loop itself around and around -- he essentially lost his mind as a boy during a Bran visitation, just as Bran was about to be overrun by a scrambling army of wights rushing down a tunnel from the weirwood tree.

“Hold the door, hold the door,” Meera (Ellie Kendrick) tells Hodor in present time as the wights try to push past the door and capture, then kill Bran. In past time, Wylis hears the command, then essentially he becomes the command. Past became present. Present became past. Wylis became Hodor, as Hodor held the door.

Talk about your circles of time -- fans already have. Books, I imagine, have even been written by now.

In fact, this one crucial idea did emerge from the mind of Martin, as “GoT” co-exec producer David Benioff said in the postgame show Sunday:

“We had this meeting with George Martin where we’re trying to get as much information as possible out of him, and probably the most shocking revelation he had for us was when he told us the origin of Hodor, how that name came about. I just remember Dan [Weiss, also co-exec producer] and I looking at each other when he said that and just being like “holy [expletive].”

advertisement | advertise on newsday

As reveals go, Sunday’s was bigger than the return of one Jon Snow. This shoves Bran into a central role in the series, perhaps the central role -- essentially, or at least potentially, the author of all that happened in Westeros, or all that ever will happen. If he can change the past, then why not change everything in the present?

There’s another looming question -- how will Hodor’s origin story be reconfigured in Martin’s forthcoming “Winds of Winter?” Surely Hodor will be holding a different door in the book -- or maybe Martin has something entirely different in store for our large, monosyllabic friend.

“GoT” handed the director’s reins for this particular episode off to Jack Bender. Bender is one of the greats -- his work is spread across the whole landscape of television, or at least the better, richer landscape ...”The Sopranos,” “I’ll Fly Away,” “Alias,” “Lost.”

Bender’s work Sunday was remarkable. There was also abundant homage here, to the rootstock of “Game of Thrones” -- notably “Lord of the Rings,” when Frodo, Sam Merry and Pippin, et al, are chased through the great hall of the mines of Moria by orcs swarming over the roof and columns. The final scene of “The Door” recaptured that instantly, and beautifully.

Also: When Bran is grabbed by the Night King, leaving a mark on his hand -- which also revealed Bran’s location. That has two memorable “LoTR” antecedents: When Pippin gazes into the palantir (that crystal ball) that reveals their location to Sauron; and when Frodo is seen by the Nazgul on Weathertop. He’s stabbed, and the fate of Middle Earth then hangs in the balance.

advertisement | advertise on newsday

Bender is also one of TV’s time travel experts -- a director who knows how to render a baffling narrative trick into something that makes sense and also something that gives it an even deeper emotional resonance. “Lost” in fact was one long time-travel trick -- at its best when deployed to understand character, at its worst when used to wrap the whole shebang up (alas, the finale).

For that reason, I suspect after Benioff and Weiss marveled at Bran’s remarkable feat, they also said: Umm, now what? Their next call must’ve been to Bender.

As a narrative device, time travel has its benefits, also drawbacks. Foremost, it introduces the notion of infinite regress into a storyline, and all sorts of paradoxes that come with that.

What if -- for example -- Bran decides to travel back in time to stop something -- let’s say, his beloved Hodor’s death? But wouldn’t the circumstances leading Hodor becoming Hodor (that is, “holding the door”) remain the same right up until the instance of Hodor’s death? Bran would then return over and over again, only to witness Hodor’s death over and over again.

Time travel paradoxes are rabbit holes, and probably not the sort of holes “Game of Thrones” (or Bran) will be heading down again anytime soon. “The past is already written, the ink is dry,” Bloodraven told Bran a couple of episodes ago. I may speak for all of us when I say: Thank goodness for that.

Nevertheless, keep an eye out for those asterisks. What if the past isn’t already written? What if the ink is of the disappearing variety?