Prequel
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60 years before World War II, mankind found ten alternate worlds that exist but with different other physical laws. The force that supports these alternate worlds is called Concept. At the same time, mankind also discovered that all these alternate worlds are about to collide and the ones without the most amount of Concept will be annihilated. To survive, a world call Low-G, which has no Positive Concepts of its own, started the Concept War to try to secure the most amount of Concepts. It took all the other worlds', called Gs, positive concepts and destroyed them all. All that remains is Low-G. Now 60 years later, which is the present, Low-G is facing destruction because of its Negative Concepts are rapidly expanding. To resolve this, Low-G needs Positive Concepts from all the other alternate worlds. UCAT, the team that destroyed the alternate worlds 60 years ago formed the Team Leviathan to begin negotiation with the survivors from all the other worlds. Mikoto Sayama is the protagonist of the story. After his grandpa, who was a key figure in the Concept War and in the negotiation team, passed away, IAI, UCAT's mother branch in Japan, explained everything to Mikoto and had him take over the negotiation seat. Carrying the phrase "The family name Sayama is villain itself" and all the hatred, young Mikoto begins his negotiation with the other worlds.
Adaptation
In the far future, humans abandon the devastated Earth and move to the upper world "Tenjo". But Tenjo and the law of causation collapse by a war and humans returns to the Earth, which has turns into an inhabitable planet except for Shinshu area. Shinshu is too small to accommodate all the humans, so they duplicate the area and create "Juso Sekai". In order to rebuild Tenjo and the law of causation, the returned humans start reproduction of history from B.C. 10,000. When the reproduction completes until A.D. 1413, a war breaks out in Shinshu and Juso Sekai falls onto the original world. The humans living in Juso Sekai lose their land and invade the original world. People in Shinshu surrender and is divided by the invaders from Juso Sekai. They try to resume the history reproduction from A.D.1457, but the update of history terminates in A.D.1648. A rumor of apocalypse begins to spread around the world.
Side Story
This side story follows Asama, Kimi and Mitotsudaira aboard Musashi one year before the beginning of the main story, as Asama is considering to create a music band together with her two friends for the next festival in Aki, a territory under the governance of K.P.A. Italia. **Note:** An official comercial release of the side story volumes of Kyoukaisenjou no Horizon that were bundled with the first press release of the 1st, 3rd, 5th and 7th Blu-ray volume of both seasons of anime.
Spin-off
The story is set in a distant future where Japan is divided into feudal territories and conquered by other countries. There is a floating city named Musashi that is composed of eight ships in the skies. A war that will determine the fate of the world will be fought by students on the Musashi. A series of humorous shorts supplementing the Kyoukaisenjou no Horizon anime.
