‘Doctor Who’ finale sees Ncuti Gatwa’s 15th Doctor regenerate into a very familiar face, but what does it mean for the show?

The Doctor is dead. Long live the Doctor!

The era of Ncuti Gatwa’s Fifteenth Doctor came to a surprise end on Saturday night, as the Time Lord regenerated at the end of “Doctor Who” season 2 finale “The Reality War”.

Massive spoilers ahead if you’re yet to watch “The Reality War”. You have been warned!!!

With Time Lord supervillain Omega dispatched without hassle, both Ranis out of the picture, and reality on the way to being restored, the Doctor gradually realises that not everything is back to normal. Poppy, his daughter with Belinda Chandra in the “Wish World” fantasy, has been erased from history, so the Time Lord decides to sacrifice himself by firing a ton of regeneration energy into the time Vortex to “jolt it one degree” — and hopefully bring her back.

It goes without saying that his madcap scheme saves Poppy, as we learn that, in this rewritten timeline, the little girl was always the reason Belinda had been desperate to get back home. But arguably the biggest talking point of the episode — and, indeed, the season — is saved until last, as the Doctor regenerates into a very familiar face…

Who does the Doctor’s new face belong to?

That’s Billie Piper, the star of “Secret Diary of a Call Girl”, “I Hate Suzie”, “Mansfield Park” and the upcoming second season of “Wednesday”.

Much more relevant right now, however, is the fact she played the Doctor’s companion, Rose Tyler, alongside Christopher Eccleston’s Ninth Doctor and David Tennant’s Tenth Doctor during the phenomenally successful first two seasons of the show’s 2005 reboot.

She’s set to become the second female incarnation of the Time Lord after Jodie Whittaker’s Thirteenth Doctor. Or the third if you count Jo Martin’s Fugitive Doctor — though the explanation of how she fits into the Doctor’s history is a little bit timey-wimey.

What happened to Rose Tyler?

After two seasons of adventuring through space and time with the Doctor, Rose was trapped in the parallel universe where the Cybus Industries had created the Cybermen. She was presumed dead back home.

The Tenth Doctor subsequently used the power of a supernova to make final contact through a tiny gap in the universe, “burning up a sun just to say goodbye”.

During their emotional farewell, Rose told the Doctor, “I love you”.

“Quite right, too,” he replied. “And I suppose. If it’s my last chance to say it. Rose Tyler…” And then the message cut off, leaving his answer a mystery — though he definitely shed a Time Lord tear or two back on the TARDIS.

That wasn’t the end of Rose Tyler’s story, however, because she managed to find a way back to her home universe to help the Doctor fight the Daleks in series 4 finale “The Stolen Earth”/”Journey’s End”.

What’s so special about Rose?

The fact she was the first of the new “Who” companions gives Rose a special place in many “Who” fans’ hearts, and Piper’s performance set an extremely high bar for every subsequent TARDIS sidekick.

But there was always the sense that this was one Doctor/companion relationship where the affection ran both ways. The “Meta-Crisis Doctor” — a quasi-regeneration of the Tenth Doctor who was essentially human — even settled down with Rose in the aforementioned alternative universe.

Rose has a particularly deep connection to the TARDIS, having looked directly into its Time Vortex in series 1 finale “Bad Wolf”/”The Parting of the Ways”. This briefly transformed her into the omnipotent Bad Wolf entity, but — despite effectively becoming a god — she’d absorbed too much energy for a human body to handle. The Ninth Doctor saved Rose by absorbing the Vortex energy into himself, triggering his regeneration into the Tenth Doctor.

It was also no coincidence that Rose Noble (Yasmin Finney), daughter of later Tenth Doctor companion Donna, chose Tyler’s name for herself after she came out as transgender — although all of Donna’s recollections of her time in the TARDIS had been wiped, some residual memories had been passed down to her daughter.

Why has the Doctor regenerated into someone we’ve seen before?

That’s the big mystery, though it’s not the first time the Doctor has regenerated into a familiar body.

Most famously, David Tennant returned to play a Fourteenth Doctor who was the spitting image of the Tenth Doctor. Meanwhile, Sixth Doctor Colin Baker had previously played Commander Maxil in Fifth Doctor story “Arc of Infinity”, and Twelfth Doctor Peter Capaldi had portrayed Caecilius in Tenth Doctor story “The Fires of Pompeii”. (Capaldi also appeared in “Who” spin-off “Torchwood”, though the Doctor was elsewhere during the events of “Children of Earth”.)

“Billie once changed the whole of television, back in 2005, and now she’s done it again!” showrunner Russell T Davies teased after the big Billie Piper reveal. “It’s an honour and a hoot to welcome her back to the TARDIS, but quite how and why and who is a story yet to be told. After 62 years, the Doctor’s adventures are only just beginning!”

That sounds intentionally ambiguous. Is Billie Piper definitely playing the Doctor?

Another excellent question, and it would be unwise to make any assumptions — in fact, even “The Reality War”‘s end titles have been engineered to add to the mystery.

Although the show’s post-regeneration credits have traditionally featured the line “And introducing [insert name] as the Doctor”, here it simply says “And introducing Billie Piper”.

The omission of “as the Doctor” is unlikely to be accidental, suggesting that Davies is playing a very elaborate game with “Who” fandom.

(It’s also worth noting that Piper played a personification of an ancient Gallifreyan weapon of mass destruction called the Moment in the 50th anniversary special “The Day of the Doctor”. In other words, there’s a precedent for the star playing new and unexpected roles in “Doctor Who”.)

When will we learn how and why Billie Piper has returned to TARDIS?

Another mystery!

The BBC and Disney+ are yet to confirm if and when “Doctor Who” will return for a third season of its current iteration. Davies told Radio Times back in April that, “There’s no decision until after season 2,” so the powers-that-be are presumably waiting to see the latest episodes’ viewing figures before making a call. Hopefully, they’ll be influenced by the fact that we still don’t know why the Doctor’s granddaughter, Susan, has been calling out to him. It’s one hell of a loose end…

In the meantime, it feels appropriate to leave the final word (for now) to the Doctor-elect:

“I love this show, and I have always said I would love to return to the Whoniverse as I have some of my best memories there,” said Piper. “So to be given the opportunity to step back on that TARDIS one more time was just something I couldn’t refuse. But who, how, why and when, you’ll just have to wait and see.”

Every episode of “Doctor Who” season 2 is now available to stream on BBC iPlayer in the UK and Disney+ in the US. All of Billie Piper’s previous adventures in the TARDIS are currently available on BBC iPlayer in the UK.