Warning: Spoilers ahead for Doctor Who season 15, episode 8, "The Reality War."Ncuti Gatwa's regeneration scene in Doctor Who season 15 cast was the last time he would regularly play the Fifteenth Doctor. While I'll be very sad to see him go, I've become immediately fascinated by the actress who has been chosen to replace him. I understand some people's grievances with the casting, but it can easily make sense.

Although only Gatwa's regeneration scene at the end of "The Reality War" didn't open the door for Rose Tyler to return. That would make no sense, as Rose is currently in an alternate universe with David Tennant's meta-crisis Doctor, so Piper becoming the Sixteenth Doctor would require some incredibly creative writing to pull off convincingly. Instead, I'm almost certain there's a better explanation for Piper's surprise comeback.

The Fifteenth Doctor Channeled His Regeneration Energy Into The TARDIS

Fifteenth's regeneration sequence was pretty different

The Fifteenth Doctor wasn't injured when he regenerated, which is often the reason for the Time Lord being forced to renew himself. Instead, Fifteen voluntarily channels his regeneration energy into the TARDIS' central column, essentially draining himself of the lifeforce required for him to remain in his current form. Although his goal is to shift reality by just one degree so that Poppy (Sienna-Robyn Mavanga-Phipps) can be restored to the timeline, Fifteen's plan could have had an unexpected impact on his next face.

Although currently trapped in another reality, Rose still has an intrinsic connection to the Doctor's TARDIS. She looked into the time and space machine's heart in Doctor Who season 1, episode 13, "The Parting of the Ways," which just so happened to be the final episode for Christopher Eccleston's Ninth Doctor. Rose's actions almost kill her, as the TARDIS is far too powerful for a human mind to process, but she briefly adopts the moniker of Bad Wolf and pulls off some pretty astounding feats.

The Doctor's redirected regeneration energy could have interacted with what Rose left behind, resulting in the Sixteenth Doctor looking just like her.

Nine ultimately absorbs the excess energy from Rose and is then forced to regenerate himself into David Tennant's Tenth Doctor. It seemed at that point that the consequences of Rose's decision to look into the heart of the TARDIS had been fully dealt with, but that may not be the case. Rose's essence could still remain deep inside the blue box. If so, the Doctor's redirected regeneration energy could have interacted with what Rose left behind, resulting in the Sixteenth Doctor looking just like her.

The Sixteenth Doctor Can Act Like Rose & It Would Make Sense

Rose's personality could be logged by the TARDIS, not just her face

If Piper is indeed playing the Sixteenth Doctor, rather than there being another reason for her return, then her version of the Time Lord may not just look like her. Piper's Doctor could even act like Rose Tyler, and it wouldn't be that weird. If the Doctor has accidentally regenerated to look like Rose because of her past interactions with the TARDIS, then it stands to reason that her personality could also have been stored in the central column rather than just her physical appearance.

Piper wouldn't need to adjust her performance as Rose in order to play the Sixteenth Doctor, and the show seems to be aware of that in the earliest moments of her comeback.

"The Reality War" has sort of already confirmed that Billie Piper's new character will act similarly to Rose. Her only current line since her return is "Oh, hello!" It's ittedly not a lot to go on, but Piper's delivery is very Rose-esque in this moment. Piper wouldn't need to adjust her performance as Rose in order to play the Sixteenth Doctor, and the show seems to be aware of that in the earliest moments of her comeback. I'd quite like this to be the case, as it feels like the most logical reason for Piper's return.

"The Reality War" is Billie Piper's 36th episode of Doctor Who.

If Sixteen has Rose's personality, then Billie Piper's return sets up an incredibly interesting reunion. David Tennant's Fourteenth Doctor still exists, living peacefully on Earth with Donna Noble (Catherine Tate) and her family. So, showrunner Russell T. Davies could pen a multi-Doctor story in Doctor Who season 16, once more pairing off David Tennant and Billie Piper, as he did for the entirety of Doctor Who season 2. Davies has made it clear by bringing both Tennant and Piper back that he's a big fan of their work, so I wouldn't be surprised if a reunion is on the cards.

Billie Piper May Not Be Playing The Doctor As We Know Them

Piper's potential Doctor could be a blend of the Time Lord and other characters

Billie Piper's return to Doctor Who is the show's third gimmicky regeneration sequence in a row, and the previous two have had high-concept explanations. Jodie Whittaker's Thirteenth Doctor reverted to David Tennant, so the Time Lord could find Donna and enjoy some much-needed rest after centuries of trauma and saving the universe. Then Fourteen split into two, which allowed Ncuti Gatwa to enter the fray as Fifteen, and the process of Gatwa's transformation into Piper to have a similarly complex reveal from Russell T. Davies, if and when the show returns.

The TARDIS itself might be sharing the Doctor's body, using Rose's face, because it has access to it from "The Parting of the Ways."

If so, then Billie Piper's new role may not be as simple as her being the Sixteenth Doctor. Instead, she could be playing a spin on the Doctor rather than a traditional version of the show's main character. One example could be that Bad Wolf is back, but is sharing the Doctor's body, which would likely make Sixteen's lifespan incredibly short, and Piper may not be back around for long. Alternatively, the TARDIS itself might be sharing the Doctor's body, using Rose's face, because it has access to it from "The Parting of the Ways."

The TARDIS has already taken humanoid form before, during the era of Matt Smith's Eleventh Doctor, so this wouldn't be unprecedented. As I mentioned earlier, there is an outside possibility that Piper is playing Rose herself, and that the Doctor's changes to reality have pulled her from the universe where we last saw her. However, I think this would be quite flimsy and problematic to the franchise's larger canon. Hopefully, Doctor Who returns sooner rather than later, and the answer becomes clear.