Chris Chibnall’s Flux has crossed the threshold into Chris Chibnall’s Delirium. If Chapter One showed the first signs of fever and Chapter Two a momentary lull, Chapter Three is like waking up in the night with the full-blown screaming ab-dabs.

Once, Upon Time is one of the most dizzying and blatantly confusing episodes of Doctor Who – and I’ve sat through a fair few – which makes it tricky to engage with.

Calling the overall six-part story Flux, establishing that space and time are disrupted and having characters stricken by “temporal haze” (flitting in and out of phase from one person and body to another, and between locations and times) may excuse a lot of seemingly random events, but there has to be the hope of coherence, a glimmer of internal logic. Of course, there can be pleasure in obfuscation and rewards from delayed explanations, and this instalment does start to make sense towards the end, maybe because the standard human brain has jettisoned much of the nonsensical detail that has gone before.

At a time when Doctor Who needs to broaden its appeal, alienating its potential audience rather than drawing it in is a risky business. Viewers poised between Countryfile and the Strictly Results show couldn’t be blamed for fretting that their BBC One signal is on scramble or for turning elsewhere.

Devoted fans will probably derive satisfaction from sifting through and unpicking this kaleidoscope of images and information, especially the notes on the Doctor’s past lives, forgotten deeds and involvement with “the Division”. Followers who abhorred The Timeless Children (the series 12 finale in 2020), especially the dismally “Mastersplained” new backstory for the Doctor, will likely throw up their arms in horror as those missing chunks of the Doctor’s past are given further currency and legitimised by the events of Once, Upon Time.

But this is where some of the gems lie. One glistening jewel is the unexpected return of “Doctor Ruth” (the “Fugitive Doctor” as she is billed), the past incarnation played majestically by Jo Martin. Any excuse for another glimpse of her, please. The same can be said for Barbara Flynn, who elevates everything she touches. She only has one scene, as the enigmatic Awsok, some superior being, possibly a Time Lady (wielding a screwdriver, note), who teases the Doctor with snippets about the origins of the Flux. Let’s have her back soon.

Jodie Whittaker sails through all these encounters and is on top of the confusion, but I’d love to have been a fly on the wall when the cast first received this script. “Um, who am I now, where am I now and what is this all about…?” perhaps. It’s hard to pin down Yaz (Mandip Gill) and Dan (John Bishop) in this piece. Some of the time they aren’t quite in character or are possibly avatars for other people.

There’s sharper focus on Vinder, the lost commander. I like his look, his changing hairdos, his jackets… he can even rock fingerless mitts. Jacob Anderson maintains sensitivity and charisma, even while Vinder’s backstory becomes staggeringly dull. He’s engaged in a crisis of conscience alongside a subdued Craig Parkinson (from Line of Duty) as the Grand Serpent (more of a Limp Grass Snake) and two bored-looking non-speaking-artist aliens.

It’s charming when Vinder sends his love message across time to Bel. Appealing played (and narrated) by Thaddea Graham (from Netflix’s The Irregulars), she reminds me of the lone fugitive types that used to populate Blake’s 7. There’ll surely be more of their fractured romance as Flux proceeds.

The delirium calls forth hovering gold Daleks, easily picked-off Cybermen (one of whom is unexpectedly talkative; nice work for Nicholas Briggs) and a flutter of Weeping Angels. Chibnall has certainly delivered some banging cliffhangers. And Chapter Three’s hits home as the Angels penetrate the Doctor’s inner sanctum.  “The Angel has the Tardis!” she cries, and it is deeply chilling. Roll on, Chapter Four, which with any luck will be a blinking nightmare. In a good way.

• Catch up on all the Jodie Whittaker adventures in the Radio Times Doctor Who Story Guide

Read more about Doctor Who:

• Doctor Who – The Halloween Apocalypse review
• Doctor Who – War of the Sontarans review
• Doctor Who: Once, Upon Time’s 8 biggest questions
• Is Doctor Who still hiding a secret regeneration?
• Is Doctor Who’s Once, Upon Time a pilot episode for a Fugitive Doctor spin-off?

Doctor Who airs on Sundays on BBC One. For more, check out our dedicated Sci-Fi page or our full TV Guide.