For months, Doctor Who fans have been pondering the mystery of “swarm,” the one-word clue dropped by series showrunner Chris Chibnall when asked when we should expect from the upcoming series 13.


Was it an alliance of monsters, possibly using an acronym? Was it some sort of alien disease? Or was it a classic monster returning in triumph? For a long time, we’ve just not known – not that that’s stopped us dropping a few wilder theories of our own.

But now, it seems the mystery is (partially) solved thanks to new episode details for Doctor Who: Flux’s first episode The Halloween Apocalypse. Alongside a new synopsis and an airtime (set your alarms for 6:25pm!), a cast list reveals that Swarm is a character – and specifically, a character played by recently-announced series 13 casting Sam Spruell.

Sam Spruell
Sam Spruell (Getty Images)

Sadly, that leaves a few of our favourite theories lying in the dust – but there are still a few mysteries left unsolved. Who, exactly, is Swarm? And have we already seen him crop up in the trailers?

Doctor Who

Some fans are already speculating that the unnamed spiky monster on the right of this picture could be Spruell in make-up, though the prosthetics are so extensive it’s difficult to tell at this stage. Notably, his female counterpart doesn’t seem to appear on the cast list for the episode (unless her name is "Diane Curtis", which would be amazing), but given that many monsters and characters are set to appear across multiple episodes this series, it doesn’t completely rule him out.

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Alternatively, it could be that Spruell’s Swarm is another character we’ve had confirmed for episode one. Previously, new pictures revealed that familiar alien soldiers the Sontarans were returning with a redesign for the series premiere – who’s to say that Spruell couldn’t be playing one of them?

Sontaran redesign in Doctor Who: Flux
Sontaran redesign in Doctor Who: Flux BBC

Swarm sounds a little strange as a Sontaran name, but the series has some precedent with S-names for this species (General Staal, Commander Skorr, Strax) so it’s not completely out of the question. And who’s to say it couldn’t be Spruell buried under that Sontaran makeup? The only downside is that he’s probably a little too tall for the role.

Of course, it’s entirely possible that neither of these theories is right, and Spruell is playing a character (possibly even a human) who has yet to appear in any teaser footage. Still, given that Chibnall chose his name as a series 13 clue, it seems likely he’ll play a crucial role in Flux so we’ll be keeping our eyes peeled for him when The Halloween Apocalypse comes to our screens.

Looks like there’s one mini-Swarm mystery still to solve after all.


Doctor Who returns to BBC One on Sunday 31s October at 6.25pm. For more, check out our dedicated Sci-Fi hub or our full TV Guide.