The face looks familiar, but who's the other Time Lord in 'Doctor Who: The Story & the Engine'?

Reverse the polarity of the spoiler flow! Proceed with caution if you're yet to watch the new "Doctor Who" episode "The Story & the Engine".

In "The Story & the Engine", the Doctor stops off for a haircut in Lagos, Nigeria, and ends up with a little more than a short back and sides. A mysterious barber (Ariyon Bakare) is on the hunt for stories to power his scheme to topple the gods, and nobody has more memorable tales to share than Gallifrey's most famous export.

The episode references most of the Doctor's familiar faces from their many adventures through space and time, along with another, less-talked-about incarnation from a previous life. Here's how the so-called Fugitive Doctor fits into "Doctor Who" lore.

The Fifteenth Doctor briefly has a different face in Omo's barbershop in "The Story & the Engine". Who is she?

That's actually a flashback to a previous regeneration of the Doctor, played by British actor Jo Martin. Known as the "Fugitive Doctor", she made her first appearance in the series 12 episode "Fugitive of the Judoon" (2020) and returned a few more times during Jodie Whittaker's Thirteenth Doctor era.

It was this incarnation that Abena (Michelle Asante) remembers. The daughter of the trickster god Anansi, Abena spent her entire life terrified that her dad would lose her in a bet, and still bears a grudge that the Doctor couldn't take her away.

"I was a fugitive back then," explains the Fugitive Doctor. "Anansi was wrong to offer that bet. And frankly, darling, I was busy in a different story that might be finished one day."

Hang on, have we missed something? We don't remember Jo Martin being the lead in any previous seasons of "Doctor Who".

That's correct, she's never been the lead Doctor in any of "Doctor Who"'s 62 years on TV. The Fugitive Doctor fits into the Time Lord's timeline before William Hartnell's First Doctor started the character's televised run of regenerations.

Are any other Doctors missing from 'Who''s traditional numerical sequence?

Yes, the long-standing numbering system has been extremely confusing since the 50th anniversary special "The Day of the Doctor" introduced the War Doctor (played by John Hurt). This version slots in between the Eighth Doctor (Paul McGann) and the Ninth Doctor (Christopher Eccleston), meaning that David Tennant's Tenth Doctor should more accurately be described as the Eleventh Doctor.

A further complication comes from the fact the Fourteenth Doctor was also played by Tennant until he regenerated — or, technically, bi-generated — into Gatwa's Fifteenth Doctor. There's also the Richard E Grant version who appeared in 1999 Comic Relief special "The Curse of Fatal Death" and became part of canon when the Doctor showed off some previous incarnations in last year's "Rogue". And that's before you've brought the Fugitive Doctor into the equation.

That said, it's worth remembering that this numbering system is more for the benefit of fans than for characters within the "Doctor Who" universe. Whatever their appearance, the Doctor is the Doctor.

Why had nobody talked about the Fugitive Doctor before "Fugitive of the Doctor"?

Because all memories of that pre-First Doctor life had been erased. Indeed, when the Thirteenth Doctor meets the Fugitive Doctor for the first time, both are confused because neither has any recollection of the other.

Why is the Fugitive Doctor so controversial within fandom?

Jo Martin's performance has been widely praised, but the Fugitive Doctor's existence fundamentally changes the 'Who' canon. TL;DR version: The Fugitive Doctor is at the heart of one of the biggest, and most divisive, retcons in the show's six-decade history.

Series 12 finale "The Timeless Children" revealed that the Doctor was never just an ordinary Time Lord who stole a TARDIS. Instead, they started out as a mysterious alien kid discovered on an alien world by a scientist named Tecteun. Tecteun was a Shobogan (the previous residents of the Time Lord homeworld, Gallifrey), and she realised that this "Timeless Child"'s ability to regenerate could be grafted onto her own species. This subsequently gave birth to the Time Lords, making the Doctor integral to the origin myth of an entire civilisation.

The Doctor was subsequently recruited to work for the Division, the shadowy Gallifreyan spy outfit that the Fugitive version is trying to quit when she first encounters the Thirteenth Doctor. It's therefore safe to assume there are many other previous versions of the Doctor that we haven't met yet.

Many fans had hoped this tectonic shift in "Who" mythology would be quietly forgotten in the show's latest iteration — much as nobody really talks about the 1996 TV movie's revelation that the Doctor is "half human on my mother's side". It's still, however, very much part of canon. The Doctor referenced being abandoned by his parents in last year's "Space Babies", and also discussed being an orphan with Ruby Sunday.

Does the Fifteenth Doctor now recall his adventures as the Fugitive Doctor?

This is an intriguing question. In "The Vanquishers" (the finale of series 13, aka "Flux"), the Doctor is given a fob watch that looks a lot like the Chameleon Arch the Tenth Doctor used to masquerade as a human teacher in classic two-parter "Human Nature"/"The Family of Blood". It contains all of the Time Lord's forgotten memories, but she opts to hide it away, telling the TARDIS to "Keep this safe, somewhere I can never find it."

However, the fact that the Fifteenth Doctor recalls his previous encounter with Abena in "The Story & the Engine" suggests that he's since downloaded these memories into his brain. Maybe he's also hoping for the Fugitive Doctor's story to be "finished one day".

New episodes of "Doctor Who" stream on BBC iPlayer in the UK and Disney+ in the US every Saturday. The Fugitive Doctor's previous appearances are available on BBC iPlayer in the UK and Max in the US.