The Doctor! Or rather, a much earlier version of her. As revealed by The Master (Sacha Dhawan) in The Timeless Children, the founding myths of Gallifrey were all lies. In reality, a traveller from Gallifrey called Tecteun discovered a young girl under a gateway, described as “a boundary into another unknown dimension or universe.”

It’s assumed that the young girl was “thrown through, seemingly, from the other unknown realm,” and Tecteun adopts the foundling and (after some other travels) ends up taking her back to Gallifrey.

But the child had some secrets to yield. After falling off a cliff while playing with a friend, the child regenerates into a new form – and upon seeing this, Tecteun becomes obsessed with understanding and replicating her ability. After many years (and regenerations for the child) she nails it, regenerating herself and eventually splicing this ability into every Gallifreyan, spawning the sub-species of Time Lords (they also, separately, discovered time travel).

Altogether, this means that the Doctor is the source code for every Time Lord in existence – the ability to regenerate wouldn’t exist without her. But her wider past remains unknown, including where she’s originally from or why she had the ability in the first place.

Who are the Division?

Also introduced in series 12, the Division are presented as a kind of Time Lord Black Ops unit, working behind-the-scenes to influence the universe despite the Time Lords’ strict non-interference policy.

In Fugitive of the Judoon, we meet one of their operatives (Ritu Arya’s Gat) and learn that an earlier version of the Doctor (played by Jo Martin) used to work for them, before eventually going off-grid and hiding as a human to escape them.

Later, in the series 12 finale we see the Doctor joining the Division, a history hidden under disguised memories of a young Irish man (remember Brendan?) who devotes years of service to the cause of law and order before being subjected to a strange, painful memory-wiping procedure. The Master suggests these Brendan scenes were a kind of code sent to the Doctor by her adoptive parent Tecteun, and they might offer clues as to her true self.

More generally, it’s implied the Division’s methods are morally dubious and violent, and the group may have died out before the Doctor’s “current” series of regenerations began – but because they use time travel, there’s no reason to think they couldn’t still bother Jodie Whittaker’s current incarnation as well.

Why is the Doctor missing memories?

It’s not been shown directly, but the Timeless Children heavily implies that after years of working for The Division (and presumably some time after the life of the Jo Martin Doctor) the Doctor has her memory wiped and is forced back into the body of a child, knowing nothing of her life before that point.

This means that for the first time, the Doctor doesn’t know her whole past, and in 2021 New Year’s Special Revolution of the Daleks she vows to investigate, finding out more about her former life and what she’s ultimately responsible for.

As noted, clues were given in visions of an Irishman named Brendan (above) who joins the Guards (the Irish police service), only to be betrayed by his father and mentor and have his memories removed when he leaves his job.

Why does the Timeless Child matter?

It’s been confirmed that the storyline will be picked up in series 13 (aka Doctor Who: Flux), and it raises all sorts of intriguing possibilities, as Whittaker’s Doctor said herself during the Timeless Children.

“Were you me all that time ago?” she says to a projection of Jo Martin’s earlier Doctor. “Were all my memories of you erased?

“How many more of me are out there?”

These questions, presumably, are what we’ll see answered before Whittaker regenerates into a new Doctor in 2022. It’s possible we’ll see the return of Martin’s Doctor, alongside other “lost” incarnations (the so-called Morbius Doctors, pictured, got a nod in the series 12 finale) as well as some big reveals about where the Doctor’s actually from.

In other words, it’s more or less the endgame for the Jodie Whittaker/Chris Chibnall era of the show. Though how it fits in with the Flux, the Ravagers, the Weeping Angels and the Sontarans is anyone’s guess.

Doctor Who: Flux airs on BBC One on Sundays. For more, check out our dedicated Sci-Fi page or our full TV Guide.