Doctor Who: Flux has proven to be an invigorating shake-up of the storytelling format of Doctor Who. Separate encounters with some of the Doctor’s most formidable enemies are set against the backdrop of an universe-threatening fire cloud and two crystal-headed rotters with plummy English accents, leaving the Doctor with a lot on her plate, all at the same time.
As with most Who-ish things, this format is by no means without precedent, especially within the classic run. And while more recent story threads such as ‘The Impossible Girl’ and ‘That Time Pregnant Amy Pond Was Abducted and Replaced By a Ganger’ show that the show can easily sustain long story arcs carried over the traditional episodic format, it’s interesting to note the similarities between this season and those long-form epics of the past.
Here are a literal handful of examples, presented in reverse order of total length, to put "Flux" into context:
5. “The War Games” – 10 x 25 minutes
 The Second Doctor’s swan song took a long and involved 10 episodes to tell its story, which involved warriors from all over time and space being stolen and placed into conflict zones for entertainment purposes. The Doctor had to traverse the various areas, find out who was responsible for creating this atrocity exhibition and put a stop to it.
This was a long single story – a firm favorite among fans – which ended with the Doctor summoning help from his race, the Time Lords. They were seen for the first time, and very quickly made their presence felt. They consented to take everyone home including the Doctor’s companions Zoe and Jamie, and then made him suffer the indignity of a forced regeneration for his trouble. It was a fittingly grand end to the Second Doctor’s era, and broke new ground in our understanding of the Doctor’s personal history.
4. “The Daleks’ Master Plan” – 13 x 25 minutesThere’s a slight quirk in the running order of this epic, in that the stand-alone story “Mission to the Unknown” was shown as a precursor to the events in this 12 episode arc, with a whole other adventure – “The Myth Makers” – sandwiched in between. Certainly the story’s writer Terry Nation was clear that “Mission to the Unknown” should be considered part of the tale, but even if it’s discounted, 12 episodes of Skaronian unpleasantness is still plenty to be going on with. And it is the longest single adventure – that is, a self-contained story, not an umbrella narrative covering several shorter stories – in Doctor Who’s history.
How long is it? Well, let’s put it this way, it’s so long that there was time to develop the plot in which the Daleks attempt to take over the solar system in the year 4000; kill off one companion; introduce a second; take Christmas Day off to generally gad about and for the Doctor to wish viewers at home a happy Christmas right down the camera lens; and kill off the second companion too, without anyone feeling unduly flustered.
3. “Doctor Who: Flux” – 6 x 50 minutes (approximately)“Flux” is a bridge between the two long storytelling models, being both a series of stand-alone stories featuring classic Who villains under an umbrella story arc, and a long-form linear narrative with cliffhangers at the end of each episode. And it’s also worth noting that at the time of writing, it may not be finished. “Flux” could be even longer and we won’t know if it is until we find out.
What it has done is remove the idea of discrete events in time and space, playing out several interrelated narratives across different eras and places – battling Sontarans in the Crimea and modern day Liverpool, sending Dan and Yaz all over the early 20^th century globe – with such deftness that the only way to involve the story’s title character in everything was to literally split her into three different people.
2. “The Trial of a Time Lord” – 14 x 25 minutes (approximately)For the two season-long epics of Doctor Who, we are firmly in the territory of the umbrella plot. “The Trial of a Time Lord” sees the Sixth Doctor facing legal consequences for his actions in front of a hostile courtroom filled with his Time Lord peers on Gallifrey. Four episodic stories are then played out, using the prosecution’s testimony as a framing device. There’s “The Mysterious Planet,” “Mindwarp,” “Terror of the Vervoids,” and “The Ultimate Foe.” It takes the format of the quest narrative – see below – and gives it extra self-awareness as the Doctor has to hotly defend himself against the pomposity of his hidebound race.
And in an echo of “The Daleks’ Master Plan,” the Doctor enters the situation with one companion – Peri Brown – and leaves with another, namely Mel Bush. Depending on your view of her retconned final moments, Peri has either died or got married to Brian Blessed, a rare case of being caught between a rock and a loud place.
1. “The Key to Time” – 26 x 25 minutes And finally, this epic tale… or series of tales… follows a quest narrative akin to the search for the Holy Grail. The Fourth Doctor and his new companion Romana have been tasked with tracking down the six pieces of a mystical stone called the Key to Time, to be delivered to the White Guardian so that he can reset the universe, realigning the forces of good against evil.
These lost fragments are conveniently hidden within six consecutive stories – “The Ribos Operation,” “The Pirate Planet,” “The Stones of Blood,” “The Androids of Tara,” “The Power of Kroll” and “The Armageddon Factor,” forming an entire season. As befits such an Arthurian quest, the reconfigured key ends up looking rather like a stone with a sword-like probe sticking out of it.
There are a few noteworthy production milestones along the way. “The Pirate Planet” is the only Doctor Who story for which Douglas Adams is given the sole writing credit. ”The Stones of Blood” is the hundredth Doctor Who story to be aired, and part 4 was shown on November 23^rd, 1978, on Doctor Who’s 15^th anniversary.  
Taken as one long story, this epic tale lasts for almost 11 hours, which, if time were wool, would be only marginally shorter than the Fourth Doctor’s scarf. 
Do you have a favorite long-form Doctor Who story?