In Ikkitousen, the Kanto region of Japan is locked in the middle of a turf war between seven different high schools. Among the students of these schools are a select few who are in possession of sacred beads. These magatama harbor the souls of warriors who fought during the Three Kingdoms Era of Chinese history. Not only are these students blessed with abilities that draw from the souls they are tied to, but they are also blessed, or maybe cursed, with the fates of these warriors from the past. One of these students is Hakufu Sonsaku; a young, caring, dim-witted girl who has recently transferred into Nanyo Academy and will be living with her cousin, Koukin Shuyu. Hakufu’s arrival creates an almost immediate sense of tension due to her power as a fighter and the possibility that she may be the one who carries the spirit of the Chinese warlord, Sun Ce. The most powerful fighters at Nanyo, known as “The Big Four” are shaken by her presence and determined to stop Hakufu from achieving the goal given to her by her mother: to conquer those who challenge her and unite the seven schools.

For more than 30 years, Dariel has been a loyal soldier in the Demon King's army, having been raised by Granverza, one of the Four Heavenly Generals. However, as Dariel lacks any magical potential, he is nothing but a disgrace in the eyes of Granverza's son, Bashubaza, who fires Dariel right after assuming his father's position as a general. Stripped of a home and a job, Dariel wanders the forests, where he stumbles upon a human girl named Marika running from a monster. After Dariel intervenes to help Marika, she invites him to stay at her village in return. There, he discovers that the reason he has zero suitability for magic as a demon is that he is, in fact, a human. However, Dariel is no ordinary human. Possessing potential that far outweighs even heroes, Dariel may soon find himself involved in the humans and demons' seemingly never-ending conflict, facing against the race he once considered allies.

Drift peacefully into Neo Venezia, a city on the planet Aqua (formerly known as Mars). By the 24th century, humans have found a way to colonize the previously uninhabitable planet. As futuristic as that sounds, Neo Venezia is still teeming with rustic beauty; gondolas on wide canals and waterways are the main mode of transportation. The city itself is a faithful replication of Manhome's (the planet formerly known as Earth) Venice. To make sure that residents and tourists alike get the most from Neo Venezia's many wonders, companies offering guided tours via gondola were formed, one of which is named Aria Company. This is the workplace of Akari Mizunashi, a free spirited teenager from Manhome who is now a novice Undine (the title given to tour guides). Join Akari as she becomes intimately acquainted with other Undine, tourists, Neo Venezia's residents, and even the city itself, learning many valuable life lessons along the way, such as the wonderful truth that there are such things as manmade miracles.

The countdown to extinction begins on Sunday with the arrival of the Septentriones, otherworldly invaders set on the eradication of mankind. Caught in the crossfire, Hibiki Kuze and his friends join in the war for humanity's survival by signing contracts with demons to become "Devil Summoners." Soon, their abilities attract the attention of JP's, an underground agency led by Yamato Houtsuin. Once recruited into JP's, Hibiki and his friends fight and bond alongside other ordinary citizens who are Devil Summoners. However, with each new day, another Septentrione appears to wreak havoc upon Japan. Even if many lives are lost in the process, before that night ends, the young summoners must defeat the invaders at all costs. [Written by MAL Rewrite]