In bold are the significant additions to my previous one to those curious what I only added.
Revised Kyoukai Senjou no Horizon Episode 1 Plot Summary
At the beginning of the episode,
we see a book being open, which could be the book, "Testament," that's later referred to in the ending credits. Next, something happens within the Earth's atmosphere (?), and then, in the next following shot, we see the camera moving upwards to the sky and land undergoing some sort of transformation. It's interesting to note because this is could be when the land in the skies that would fall soon after, not the Earth's land transforming. And I believe this to be so, which I'll explain later. This may also be what is later suggested in the conversation between the two teachers, Musashi and Sakai,"the apocalypse." After the opening, we see a girl singing at a grave with the name Horizon inscribed on this gravestone. Later, throughout the episode, it could be implied or suggested this girl is the sister of Horizon. Probably Horizon's imouto. The next scene transitions to our main character, Tori Aoi, who offhandedly remarks something needs to be put at an end.
And this character is/will be important because the following exposition between the instructor and the boy with the glasses sets up Tori's background. Apparently, Tori is the representative/Chancellor of this flying city/ship called, "Far East Musashi." Reps are usually chosen by the "Testament Union," and they usually pick incompetent fools/puppets basically, so they can easily manipulate them, including Tori Aoi, our main character. It also provides us with some context and even motivation for his character. Our main character was once incapable seemingly. Is he still now? That remains the question onto the next episode.
One important thing to note is that the "Far East Musashi" is basically a flying estate/flying ship - the ship Tori and his ragtag team of classmates (and friends?) are on now. Because the Earth only has a few remaining inhabitable pieces of land, which is practically the size of Japan (is Japan!?), more or less, as indicated in the following picture is divided into separate states/countries is called, "the Divine States."
Here's the catch: all territories - the Divine States - previously belonged to Far East Musashi, but they're oppressed for reasons I'll detail later. Think of Far East Musashi as Britannia basically. The age limit requirement imposed on them is an example of these politics.
Before P.E. begins, there's another important thing you should remember. The instructor specifically says after passing Horizon's gravestone that Horizon marks the start of their journey and cuts to a shot of the class chasing after her. This declaration could also imply all of them were friends or fondly knew of Horizon before.
Anyway, near the end, Tori shows up and proclaims he'll finally have a confession. And while it's unknown what a confession was initially, when he states it's a confession to Horizon, who Tori's sister Kimi Aoi/Bel Flore says died 10 years ago, we can safely assume they're implying this is a love confession. Also, the name Horizon has meaning because of the gravestone shown earlier in the episode. We also learn something important about Tori: he never confessed to Horizon and plans to do so before he challenges the world. Tori might be suggesting he's planning to lead a revolt with Far East Musashi and his classmates.
Now here's where most of the confusion is: the exposition in the ending credits. From what I can interpret, they're trying to rewrite history because they want to return to their "former prosperity" - which I think is to prevent the collapse of their former land, the land in the skies, as I mentioned at the beginning of this summary. As the ending credits state, humanity once rose to the heavens, and from this statement, I believe humanity before lived on land they formed above the Earth, floating in the skies. However, a war up there caused the land in the skies to collapse as seen perhaps at the very beginning of this episode, and they were forced to return to the Earth. The problem was most of the land was uninhabitable. And what is known as the "Divine States" was the only land left habitable. So in an attempt to fix this, a return to their "former prosperity" - to prevent the destruction (the same apocalypse maybe?) of the land in the skies, they set forth to rewrite/relive history by creating a magic book, "the Testament," where they can literally replace the present with a newly created future within the book where the land in the skies never collapsed, thus, preventing it from ever happening. I like to describe this method/fixing as a sort of mechanic like time travel except they never explicitly state it's time traveling although it involves time manipulation.
However, they failed once already.
So here's where the real confusion starts: for simplicity's sake, the present where the land in the skies collapsed is "the Divine States," and they created copy to undo their fuck ups so they can do a reset, "the Harmonic Divine States." You can also think of this as two dimensions. What happened was the Harmonic Divine States fell because of their failure, and as I would like to describe, the Harmonic Divine States merged with the Divine States creating one timeline/one dimension. What the people from the Harmonic Divine States did, however, was invade the Divine States and conquer the Divine States/Far East/Japan and divided the land amongst themselves, separated into states/countries as see in the picture above, and segregated the Far East people, which includes the flying ship, Musashi.
Now let's reconnect this information from the exposition they gave earlier at the beginning of P.E. As the boy with glasses says to the instructor, the Divine States use to belong to them, the Far East people, but it was taken from them. It's reasonable to assume the Divine States use to also be called the Far East. As the picture above helps clarify, there are other Far East people scattered around the Divine States, and it would help explain what the narrator meant or is implying with the warriors of the Far East being scattered around the Divine States. I wonder if Tori gets to recruit heroes and stuff.
The most intriguing thing I guess is how all this setup is a reenactment of the Japanese Warring States period war with Tori and his friends (?) since they're in that time where it took place. It could imply something too but eh... I won't go into it.
In short, this is some complicated justification for a setting than any sort of plot :lol