Doctor Who: Series 13 Trailer Breakdown - 20 Things You Must See

Assuming that this line is referring to the Flux, then that last word - "again" - is really quite interesting, isn't it?

Of course, this could be a totally unrelated line plucked out of context and slotted into the trailer, but another line we hear is the Doctor saying "Why don't I know about this... and who else does?", which implies that the Flux is a known event that other people are aware of - like, perhaps, this random old woman.

So what is the Flux? And how could it happen again? And how could the Doctor not know about it? Well, based on those broken-up lines used in the trailer's title cards (see below), it seems pretty clear that the Flux is something to do with various timelines going all wibbly-wobbly.

And if that is the case, then the Doctor might not know about it because she's a time traveller, and time doesn't have the same effect on her that it does on other people.

Here's an idea: what if the Master's destruction of Gallifrey (and annihilation of the Time Lords) has caused time itself to become unstable? Maybe something similar to the Flux happened during the Last Great Time War, with the ripples of that war being felt across the universe (hence why the old lady says "it's happening again").

By messing with timelines, Chibnall also has an easy way to bring back Gallifrey, while also possibly addressing some of those Timeless Child criticisms.