Everything old is new again this year on Doctor Who. But what else would you expect of the Doctor's timey-wimey ways?

Season 9, which premieres Saturday at 9/8c on BBC America, is returning to the old formula of paired episodes. A story will start and end with a cliff-hanger one week, and then conclude the following week. The season consists of 12 episodes proper — six two-parters — plus a Christmas special to make a total of 13.

"This is the most two-parters we've had. I wanted to shake up the idea that it ends after 45 minutes," executive producer Steven Moffat tells TVGuide.com. "It's also to blur the distinction between what is a two-parter and what are two linked stories. So, you'll never quite know how much of the story we're going to finish in 45 minutes."

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Before we dive into what we know about each of those 13 episodes, we need to mention that recently, it was reported that Jenna Coleman, who plays companion Clara Oswald, would be leaving the series after Episode 12. BBC America has yet to comment on the exit, but Coleman confirmed it on BBC1 Radio Friday. "I have left the TARDIS," Coleman told host Nick Grimshaw. "It's happened. I filmed my last scene."

The show has already begun on the Christmas episode, possibly sans Coleman. Clara was the companion to Matt Smith's 11th Doctor for one season before he left, and with Peter Capaldi's 12th Doctor in his debut season last year (after which Coleman had intended to leave but then changed her mind). Although Clara made a positive impression on viewers when she first appeared in her "Soufflé Girl" and "Victorian Clara" personas, her modern-day character never inspired the same level of devotion in fans. It remains to be seen if she leaves, who would become Capaldi's new companion(s).

But we're getting ahead of ourselves. An entire season full of monsters, time travel and wacky Doctor fashions lay ahead. Here's the breakdown:

Episodes 1 and 2: "The Magician's Apprentice," and "The Witch's Familiar"

Besides looking older, the 12th Doctor never really distinguished himself in any sartorial way last season. This year, the Doctor's hair is longer and more unruly, and he's developed a habit of wearing sunglasses. Perhaps it has to do with his new rock star look when he shreds an electric guitar in the premiere. "Peter asked if he could have an electric guitar, and I went for it," Moffat reveals. "It's the sort of whacked-out thing the Doctor would do: know that he doesn't look like a rock star, but he decided to become one. He settled into his new life, and he's loosened up a bit really."

You can also expect the return of Missy (Michelle Gomez), who is more annoyingly fun than ever, and another of the Doctor's archenemies (really, he has so many). The Doctor will also travel to an alien planet beset by war, and Clara will find herself in a frightening trap. No real worries though, since the Doctor and Clara's bond is stronger than ever. Moffat says, "They straightforwardly adore each other. That's what we made them earn through the previous 13 episodes."

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Episodes 3 and 4: "Under the Lake" and "Before the Flood"

These adventures take place in what appears to be a submarine or underwater base. Ghostly beings with empty eyes haunt the corridors, and one of them has a jaunty top hat. Our best guess, judging from the title "Under the Lake," is that the story may have ties to Arthurian legend and the Lady of the Lake. This would support one report that an episode had been named for the Fisher King, an Arthurian legend about a wounded king who must guard the Holy Grail but is rendered impotent.

Episodes 5 and 6: "The Girl Who Died" and "The Woman Who Lived"

At last, we come to the arc featuring Game of Thrones star Maisie Williams as... well your guess may make more sense than ours. The only fact we are sure of is that the Doctor does not know this character from before (and that means you can forget all your theories that she's the Doctor's regenerated granddaughter Susan).

According to a Mashable interview, Williams character will also somehow be connected to Vikings on a spaceship (as one does on Doctor Who) andpossibly a dragon, at least for the first part of her arc. After going through the second Doctor Who trailer frame by frame, we'd say that would explain one of Maisie's looks — her hair braided around her head and wearing a plain tunic — but not the second one. Perhaps in the second part of this story, she inhabits a time period closer to the Victorian era, one that includes a creature that looks Beauty and the Beast-inspired (but breathes fire). See for yourself:

Episodes 7 and 8: "The Zygon Invasion" and "The Zygon Inversion"

Those red, shape-shifting zygons are back and invading Earth, which spurs UNIT into action. "I've been wanting to do this kind of story, a global, urban thriller with shape-shifters," Moffat says. "When I wrote the 50th [anniversary special], I did write that strand of the 50th as a deliberate setup for another story. We're actually going to pick up that story."

The story also somehow inspires the return of Ingrid Oliver as Osgood. Wait, didn't Missy kill Osgood? Maybe, maybe not. "It wasn't a sudden whim to bring her back, it was always the plan," Moffat says. "Everything has been set up for a particular kind of story. She's got a very big part in that. I haven't seen anyone guess how it works."

Episodes 9 and 10: "Sleep No More" and "Face the Raven"

Mark Gatiss wrote the first half, which is supposed to be the ultimate in scary. Then again, he wrote that uneven "Robot of Sherwood" episode last year, so we'll see. In the second half, the Doctor and Clara will be on Earth and reunite with the grafitti artist Rigsy (Joivan Wade).

Episodes 11 and 12: "Heaven Sent" and "Hell Bent"

Sorry, neither Moffat nor BBC America would divulge anything about these two episodes other than that Episode 11 will end on a major cliff-hanger, which is a no-brainer since it leads into the season finale. Rachel Talalay, who directed last year's season-enders, is back to do the honors again. Clearly, this could be the last episode for Clara.

Episode 13 - Untitled Christmas Episode

The Doctor's wife River Song is back! "The last time the Doctor saw her, she was a ghost," Moffat said in a statement. "The first time he met her, she died. So how can he be seeing her again? As ever, with the most complicated relationship in the universe, it's a matter of time ..."

River Song (Alex Kingston) married the Doctor in his Matt Smith incarnation, but we have yet to see her meet the regenerated Capaldi version.

Doctor Who's ninth season premieres on Saturday at 9/8c on BBC America.