The thirteenth series of the British science fiction television programme Doctor Who, known collectively as Flux, was broadcast from 31 October to 5 December 2021.

The thirteenth series also stars Mandip Gill and John Bishop as the Doctor's travelling companions, playing Yasmin Khan and Dan Lewis, respectively.

[8][9] Following the departures of Bradley Walsh and Tosin Cole in "Revolution of the Daleks" (2021), John Bishop joined the cast for the series as Dan Lewis.

[14] Also appearing in recurring roles throughout the series are Thaddea Graham as Vinder's paramour Bel, Craige Els as the Lupari Karvanista, Rochenda Sandall and Sam Spruell as the villainous Ravagers Azure and Swarm, Annabel Scholey as Claire, and Kevin McNally as Professor Jericho.

[15] Other guest actors in the series include Craig Parkinson as the Grand Serpent, Sara Powell as Mary Seacole, Gerald Kyd as General Logan, Penelope Ann McGhie as Mrs Hayward, Steve Oram as Joseph Williamson, Nadia Albina as Diane, Jonathan Watson as Commander Riskaw and Skaak, and Paul Broughton as Neville.

[20] Chibnall stated that both he and Whittaker had originally agreed to only do three series, and that "now our shift is done, and we're handing back the Tardis keys".

[25] Whittaker stated that in the series, the Doctor is "faced with a temptation of delving into the mysteries of her past", because "it's hard to go forward, if you don't know where you have come from".

He decided to start the sequence in medias res, to imply that Yaz and the Doctor have had many adventures since the end of the previous series.

[16][29] According to production executive Tracie Simpson, pre-production for the thirteenth series was originally set to begin in June 2020, with filming set for September 2020;[30] however filming only eventually commenced in November 2020, and ran for ten months,[17][31] even during the time the series trailer was dropped in July 2021 at San Diego Comic-Con.

According to him, COVID presented some "curveballs" as well, due to cases causing crew shuffling, and the need to shoot it single-camera as opposed to multi-camera.

[33] Speaking to Doctor Who Magazine, Stone and Saleem both stated that they liked working with the "Weeping Angels", because of a reversal which puts editing and negative spaces in the forefront of their terror.

He explained that since he began working with the show, each series' soundtrack had been an evolution of the previous one, with the serial format leading to an even bigger change.

[40] Speaking of Flux in particular, Akinola stated that the changes they made included a greater use of synth, which they also used in a more foregrounded way, and the use of orchestral elements in each and every episode.

[45][46] Other sources of promotion included images of upcoming guest stars,[47] episodic promos,[48] the Doctor Who Magazine, and interviews with Whitaker.

The release contained supplementary "special features", which included behind-the-scenes footage, the series trailer, a vlog and an audio commentary for the second episode.

[72][73] The serial did moderately well, finishing in the top 25 every week, achieving a maximum of being the 8th highest watched show with the eighth episode.

[5] In terms of overnight viewership (which were shared by BBC News entertainment correspondent Lizo Mzimba via Twitter), "The Halloween Apocalypse" had about 400,000 less than the series 12 premier "Spyfall", having netted 4.4 million viewers.

[86][87][88] Some reviewers also felt that the villains were built up as "big bads", but were then easily dispatched in the end; they also mention the Flux, and how it seems no one, including the Doctor, seems to care about the destruction to space and life that has been caused due to its effect.

[87][88] The serial form of the series was also noted, some disliking it and finding it to lack consistently and cohesion,[88] while others felt that it allowed Chibnall to play to his strength of interweaving and writing strong cliffhangers.

[86][89][90][91] Chris Allcock, in his review of episode six for Den of Geek states that it's "maddening" that either Chibnall does not care about the aftermath of the Flux, or has mistakenly given us that impression.

He characterised the series as containing some of Chibnall's best episodes, but still showing "a total disdain for picking up after itself", something the audience will watch once, laugh, and "forget it as soon as it ends".

Club's Caroline Siede states that even though Chibnall can write "really solid character scenes", they feel secondary due to the overcrowded plot.

He concluded by saying he found Flux funny and "full of spectacle", with some of the "best guest stars" ever in the show, with Kevin McNally and Craige Els as Jericho and Karvanista in particular.

Cleary thought the series felt anti-climatic and that the ending "doesn't pay off",[90] while Jeffery and Cook criticised the writing and lack of direction, calling it "somewhat incomprehensible".