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Doctor Whointroduced its Fourteenth Doctor on Sunday night in the UK, ending Jodie Whittaker's five-year tenure as the Time Lord's thirteenth incarnation. The Doctor's regeneration wasn't a surprise to anyone, with Whittaker's exit announced well in advance last year. We've also known since May that Sex Education actor Ncuti Gatwa would soon be stepping into the role of Doctor.

Warning: Spoilers for Doctor Whofollow.

However, an unexpected twist still managed to shock viewers, and has long-time fans absolutely ecstatic.

Though if you're a Whovian, chances are it's been all over your Twitter feed.

Written by outgoing showrunner Chris Chibnall, "The Power of the Doctor" was the last of three Doctor Whospecials that had been scheduled for 2022. The episode had the Thirteenth Doctor tangle with classic adversaries such as the Daleks and the Master (Sacha Dhawan); reunited her with former companions like Tegan Jovanka (Janet Fielding) and Ace (Sophie Aldred); and even had a few of her former incarnations poke their heads in, including the Fifth (Peter Davidson) and Seventh (Sylvester McCoy) Doctors.

Still, the biggest surprise and delight of the episode was its final scene, in which Whittaker's Thirteenth Doctor tags in the next Doctor and finally regenerates — into David Tennant, not Gatwa.

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Tennant previously played the Tenth Doctor from 2005 to 2010, with his run widely lauded by both critics and fans. Arguably the most beloved Doctor since the sci-fi series' revival in 2005, his ongoing love for the series is well known and has been widely documented. Even so, nobody expected Tennant to return as Doctor Who's protagonist, or even believed it possible under the show's lore.

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It seems we all forgot a core tenet of Doctor Who: Nothing is impossible. Just a bit unlikely.

SEE ALSO:'Sex Education' star Ncuti Gatwa named the new Doctor on 'Doctor Who'

With former showrunner Russell T. Davis also returning to Doctor Who, fans are hopeful the Fourteenth Doctor's run will be full of the same camp fun that first hooked them on the series. 

It's already offered more than one callback at the very least. Upon regenerating on Sunday, Tennant's Fourteenth Doctor immediately commented that he "know[s] these teeth," a reference to his first lines as the Tenth Doctor. He also repeated "what" several times in bafflement, a signature of the character.

The nostalgia will be further bolstered by the Tenth Doctor's former companion Donna Noble (Catherine Tate) also returning to the show. Though Tennant and Tate's returns to Doctor Who were announced in September, many fans reasonably assumed it'd be in a flashback, missing story, or even a parallel universe. Now it seems possible we could find out what the best temp in Chiswick has been up to all these years.

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Tennant's return has been widely celebrated, however it was a bit of a blow to anyone looking forward to seeing Gatwa in the TARDIS this year. Fortunately, Doctor Who's regeneration twist doesn't mean he's been shafted from the job. 

Gatwa will take up the sonic screwdriver as the series' Fifteenth Doctor after Tennant's second run, which will feature the Fourteenth Doctor in three special episodes scheduled to air in November next year. These episodes will celebrate Doctor Who's 60th anniversary. 

Notably, the May announcement that Gatwa would be portraying the Doctor stopped short of specifically naming him the Fourteenth Doctor. This omission seems rather telling in retrospect — particularly when compared to the wording in other Doctors' casting announcements. 

Of course, there wasn't enough evidence to draw any strong conclusions at the time. But it seems the careful wording left just enough of a crack for the Doctor's particular brand of mischief to wiggle through.

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