Kyoukai Senjou no Horizon is different from any other action, fantasy anime in a way that it incorporates both elements into one with comedy and romance thrown in. It is quite difficult to discern what is happening when all these elements are combined.
In the far future, humans abandon a devastated Earth and traveled to outer space. However, due to unknown phenomenon that prevents them from traveling into space, humanity returns to Earth only to find it inhospitable except for Japan.
To accomodate the entire human population, pocket dimensions are created around Japan to house in the populace. In order to find a way to return to outer space, the humans began reenacting human history according to the Holy book Testament. But in the year 1413 of the Testament Era, the nations of the pocket dimensions invade and conquer Japan, dividing the territory into feudal fiefdoms and forcing the original inhabitants of Japan to leave.
It is now the year 1648 of the Testament Era, the refugees of Japan now live in the city ship Musashi, where it constantly travels around Japan while being watched by the Testament Union, the authority that runs the re-enactment of history. However, rumors of an apocalypse and war begins to spread when the Testament stops revealing what happens next after 1648.
Taking advantage of this situation, Toori Aoi, head of Musashi Ariadust Academy's Supreme Federation and President of the student council, leads his fellow classmates to use this opportunity to regain their homeland.
Unlike you average action, comedy, fantasy and romance anime that splits all elements into different episodes, Kyoukai Senjou no Horizon tends to complicate things and mash all these elements into one episode. If you are not careful enough, you may find yourself quite lost as to what exactly is happening. That said, the story is not as simple as it sounds. Basically, the setting takes place in the future where humans are living above the ruined Earth and trying to find a way to travel back in space. To do that, they decide to follow the Testament history book. Every country is trying to re-enact history in order to find the way to travel in space, however a rumour spreads that the world will end after the year 1648. That is just the background story. The main storyline in this series is about Toori Aoi trying to save his automated doll girlfriend from the Testament Union. As like any other action fantasy anime, there are plenty of battles, different kinds of people and weapons as well as comedy in almost every form.