The BBC has announced is returning to its classic roots for season 13, because multi-episode stories were normal in the original TV series.
The Doctor Who season 13 trailer doesn't give much away, because it's presented as an emergency message from the Doctor attempting to warn Earth that "the Flux" is coming. True to form, the Doctor doesn't give any details - she simply tosses out the word "Flux" breathlessly and barrels on, leaving the term undefined - but apparently it will bring with it Sontarans, Weeping Angels, and creatures known as the Ravagers. With the Flux driving the entire narrative of Doctor Who season 13, it's being marketed under a single title: "Flux."
But the BBC may well have already dropped an important clue. Back in January 2021, when the BBC first announced the arrival of new companion John Bishop, they included an unusual comment; that he would "quickly learn there’s more to the Universe(s) than he could ever believe." The clear implication is that Doctor Who season 13 is going to involve multiple universes, and there have recently been curious nods to a multiverse in some Doctor Who marketing. The word "flux" can be used to refer to a period of continuous change, so it's possible reality itself is fluctuating around the Doctor and her fam and they are forced to navigate universes that are being rewritten around them.
Time has always been portrayed as fluid in the BBC TV series; that's why 's continuity is so messy, because it's frankly impossible to have any real sense of canon when time travelers are constantly rewriting history around themselves. The Doctor is traditionally the cause of these temporal fluctuations, but it's possible this time she's on the receiving end of them. Indeed, the marketing campaign for Doctor Who season 13 - which has featured a #FindtheDoctor campaign and the brief removal of every one of the show's social media s - may in itself be a hint; it's possible the Doctor finds herself navigating a universe where she has been erased from history.
Whatever the nature of the Flux may be, it's unlikely to be a natural phenomenon, and in the season 13 trailer the Doctor claims it "brings with it" Weeping Angels, Sontarans, and Ravagers - not that they are the cause of it. Marketing for Doctor Who season 13 has been extremely secretive, and it's reasonable to assume the real villain - the power behind the Flux, so to speak - is yet to be revealed.