When you write a show with 57 years of canon to sort through, it can be difficult to keep inconsistencies at bay. But Chris Chibnall, showrunner for Doctor Who since 2016, may have found a way to use all those thorny bits of lore to conceal the backstory of the character who's throwing the fandom for a loop.
In Season 12, Episode 5, Ruth Clayton, mild-mannered tour guide, revealed herself to be The Doctor, baffling the Thirteenth Doctor played by Jodie Whitaker. Ruth can't remember Thirteen, so obviously she can't be in Thirteen's future. But at the same time, Thirteen can't remember Ruth, so it's unlikely she's in her past as well. It seems impossible. That's where the 6B theory comes in.
The 6B theory refers to Season 6 of the original Doctor Who, when the Second Doctor, played by Patrick Troughton, was wrapping up his tenure. In his last run of episodes, the Doctor is put on trial on his home planet of Gallifrey and sentenced to exile on Earth and a forced regeneration.
The key to the theory is that unlike every other Doctor's last episode, the next Doctor did not appear in the final moments. Therefore, between the second and third Doctors, there could be an unspecified number of doctors in between, effectively in a long stretch of unseen "Season 6B" adventures, but these memories were erased later as part of the criminal sentence.
This theory originated among fans, but was soon was adopted by the BBC as a catchall to explain occasional continuity clashes with previous Doctor Who episodes. Up until 2011, Season 6B was listed on the BBC's official Doctor Who episode guide. However, it was mainly seen as a relic of the bygone era when Doctor Who ran multi-episode standalone stories.
So why is this theory making a resurgence? Reddit user CheddarCheeseCurds argues it may be an explanation for Ruth: those memory wipes would explain why she can't remember her past as a Time Lord. The Thirteenth Doctor can't recognize Ruth, but she is in her past.
There's lots of evidence to back this theory up as well: we see the interior of Ruth's TARDIS, and it looks extremely retro, not at all like the organic rock formations in the Thirteenth Doctor's. She also can't recognize the Doctor's sonic screwdriver, possibly eluding towards memory meddling. In an interview with the U.K.'s Daily Mirror, Chibnall denied that Ruth is from a parallel universe, which may lend more credibility to the 6B theory.
Fans will probably have to wait until the season finale to find out the true explanation, as the Thirteenth Doctor's got lots of adventuring scheduled before she has to reckon with this new part of her timeline.
Doctor Who airs Sundays at 8 p.m. on BBC America.