Doctor Who’s ninth season has been equal parts frustrating and exhilarating, usually within the same episode. But in recent episode “Heaven Sent,” the show has done something it hasn’t done since its revival in 2005: reinvent itself.

It's a good thing, too, because long-running shows have the potential to retread old ground. Not "Heaven Sent." Here's why.

SPOILERS for “Heaven Sent.”

You’ve been warned.

Time travel the hard way

Since its inception, the Doctor has been a runaway time traveler, and we’ve seen him at the beginning of the universe (“Nothing O’Clock” by Neil Gaiman) and at the end of the end of days (“Utopia”). In every case, he has journeyed from point A to point B in his TARDIS. But not so in “Heaven Sent,” in which he travels two billion years ... the long way around.



Time travel has always been as inconsequential as getting into a police box and flipping a lever. Seeing the Doctor count millions of years passing by the placement of the stars breaks Doctor Who's own premise. We're seeing something we've never seen before, and it's the very definition of science fiction.

Let’s get physical

In “Heaven Sent,” the Doctor dies repeatedly and is renewed again and again (reset, in a way, via a teleporter), until he breaks free of a prison made of a substance harder than diamond. With his fists.

This is another important reinvention, not to the story but as a piece of character development. In his multi-year history, the Doctor has never brute-forced a solution. To save the day, he has used sleight of hand, excellent timing and/or the sacrifices of friends and strangers. But he has never punched his way through a problem.

That the prison was on his home planet of Gallifrey, likely created by his people, is practically an afterthought. That’s how remarkable this episode is -- the Time Lords may be the enemy, but the Doctor's only focus has dwindled to the wall in front of him. The Doctor, who has always been a big-picture kind of Time Lord, changes completely in the course of one episode. What other show can claim that?

All will be revealed

The episode’s third achievement is that we finally, FINALLY learned why the Doctor stole his TARDIS and retreated from his home of Gallifrey. He did so to skirt a prophecy, a “legend before the fact,” of an “ultimate warrior” who was half-Dalek, half-Time Lord. He fled, he said, because he knew the hybrid of legend was real.

But it isn’t until he breaks free when we get the real reinvention of Doctor Who. It turns out that he knows the hybrid doesn’t exist ... but the prophecy does. “The hybrid destined to conquer Gallifrey and stand in its ruins is me.”

Now, we know that, despite all of his world-saving acts, our hero may actually be a villain. Heroes have gone rogue before, but they rarely threaten to destroy a civilization. It's a chilling thought, and it makes me almost scared to watch the season's finale (almost).



One-man show

Technically, “Heaven Sent” isn’t a one-man show. The character of the Veil wordlessly appears to torment the Doctor, and Clara appears in what is surely a hallucination and speaks a few sentences. But other that those two, plus a young boy at the finale, the Doctor spends the episode alone.

Worse, he’s held in a prison without companion or companionship, designed to scare him to death. The real result of this torture chamber is we get to see what a powerhouse of an actor Peter Capaldi is. I haven’t been this impressed with a performance in Doctor Who since Lesley Sharp’s turn as Sky Silvestry in “Midnight.”

Making sense of the fourth wall

Technically, this wasn’t the first time the Doctor broke the fourth wall and addressed the audience directly. We saw it in episode “Before the Flood,” in which he spoke about Beethoven and time paradoxes. It was an odd scene, one that was set completely in the TARDIS, much like many scenes in "Heaven Sent." which had nothing to do with the episode in which appeared .

Here, we realize that the musings in “Before the Flood” were likely a flash-forward to “Heaven Sent.” You can tell by the episode’s atmospheric score: If the music isn’t Beethoven, it’s pure Beethovenian. Its use of sound was wonderful way to tie up one of Doctor Who’s unusual loose ends.

Aftermath

In the previous episode “Face the Raven,” Clara told the Doctor before she died, “I know what you’re capable of. You don’t be a warrior. Promise me. Be a Doctor.” But with each punch against the diamond wall, we see him as warrior, and he’s destined — and now highly motivated — to destroy Gallifrey.

Will we be getting the worst version of the Doctor — a variant of the War Doctor who stopped the Time War only by killing everyone in it? The one who rejected his own name in favor of the Warrior?

We won’t know until next week’s episode, “Hell Bent.” For Whovians awaiting the fallout, this week will seem like forever. And not even the Doctor can help us.