Washio Sumi's Chapter is a prologue to Yuki Yuna is a Hero that serves as both a redefining experience and as a unique universe addition. Its goal was to sit somewhere been essential world-building and narrative, or simply being an extra story to flesh out the main series.
I say this because there's 2 ways to experience the Washio story. Released originally as a light novel, chapters were coming in between episodes and thus were made in such a way as to not spoil the original series and should probably be watched (or read) alongside the original series... to a certain extent. If you haven't already seen Yuki Yuna is a Hero or plan on rewatching it, then I absolutely recommend the watch order detailed here https://tinyurl.com/yajt5dz9
It gets even more confusing to recommend the best way to experience the Washio Sumi story, as the series was originally adapted into a theatrical run of 3 45 minute movies, that were then condensed slightly into a TV format.
It's awfully complex, considering it's for 2 hours of content. But it's a very complex and worthwhile 2 hours of content.
Overcoming the short runtime is an efficiency of focus. Almost no time is wasted, magnifying solely on 3 things: the 3 leads' lives, the battles and the world.
The condensing of material into just 6 episodes was surprisingly smooth. The introductions were fast but powerful, and the singular slice-of-life episode used a skit-style method to push all of its various ideas, including the leads lives but also to alter our perception of the world they live in, and foreshadow the rest of the series and the original series. It's creative; it's fun, and when it comes down to it, utterly distressing.
The leads go through a lot of development. They realistically and touchingly go from barely even being aware of one another to the best of friends; they heartbreakingly go from being society's outcasts to its heroes, and find something meaningful, something beautiful, along the way to fight for.
"Do your best no matter what" in the original series becomes "make sure to come back", and considering the meek lives these girls have had, and the courage they employ in spite of that, is truly heartwrenching to watch. A particular scream from Washio herself in episode 5/movie 3 is thoroughly spine-tingling, and I dread that the battle following that, with its infinite aggression and rage, may just stay with me for the rest of my life as one of the most distressing breakdowns in anime.
Make no mistake: The Washio Sumi Chapter, like its sequel series Yuki Yuna is a Hero, is one of the darkest magical girl shows around. Yet somehow, Washio Sumi's Chapter manages to eclipse the original series in how far its willing to challenge its leads, while still pushing the unique sense of tear-forming melancholy rather than stomach-churning dread.
Downsizing from 5/6 to 3 heroes makes a big difference, and the girls learn how to put their unique skills together to best fight the very specific, creatively designed vertexes, without becoming video-game technobabble or cheesy.
The movie isn't necessarily subtle about where its story is going, but I'm forever impressed by the subtlety of the world-building. Peppered throughout is a distinct idea of an entirely different, dystopian and apocalyptic world, but these are restricted almost entirely to the convention of the world - there is no idiot-dumping to get you to realise the world they live in is hell, it simply is, and everybody is finding a unique way of coping.
Perhaps in part due to its theatrical run, the visuals are gorgeous... aside from the ever-present fanservice which is just unneeded. It continues to smear the transformation sequences in an overly sexual way, and permeates even into the script as the reference Washio's breasts 2-3 times. It's a reduction compared to the original series, but it's still there and one of the biggest barriers into the franchise.
The music is typically excellent as it was in the original, and the final ED (Yakusoku/Promise) is possibly the greatest tearjerker I've heard in years (seriously, do not read those lyrics once you've got context).
I can't write a review without mentioning the excellent performance from Kana Hanazawa as Nogi Sonnoko. "IT'S KAGAWA LIIFEE" is perhaps one of the most funny, grounded, slice-of-life sequence that will forever be a top-tier meme for its honest self-awareness. "We're friends forever, you see," on the other hand, is one of the most tearjerking monologues in anime history.
(just writing those words down is bringing tears to my eyes. I'm not strong enough for this series)
In my honest opinion, unless you are going to watch the watch order I recommended earlier, and plan on watching the series as a standalone after finishing the original series, then I would recommend the Washio Sumi Chapter movie trilogy over the series. The 45 minute movies are just that little bit better at execution, most notably in the final, though there are very few differences other than the OP/ED breaks.
The Washio Sumi Chapter's addition to the franchise makes the already-impressive Yuki Yuna series even more enticing. There's significant depth added in a strongly executed string of 6 episodes, and it widens the perception of the original series in a very unique way. It helps that the Washio story itself is excellent, but it's not quite self-contained enough to be seen as a standalone. Even though it is fantastic as an additional story, it's hard to recommend the convoluted watch-order to truly experience its universe-altering story.