The world of Ar Ciel is a planet where emotions are power, sound is strength, and through singing the world can be changed. With the advent of Song Science and the creation of the three towers of Ar Tonelico, the Reyvateils were born. Genetically engineered songstresses capable of connecting to the towers to perform great acts with their singing, and the three Origins, Shurelis, Frelia, and Tyria were made to run the Towers. But all was not meant to be.
The Grathnode Inferia, a disaster wrought by mankind in their hubris to weaponize the power of Song. It was not to be, and in a great cataclysm the world was covered in the Sea of Death, a noxious cloud of toxin shrouding the planet and forcing its residents to eke out a meager living upon the three Towers, the last possible places of refuge, as the skies were bathed eternally in the Sea of Plasma, born from the same disastrous event.
A new age was born, which man and Reyvateil co-existed in a power struggle. Humanity, still arrogant, sought to enslave the Reyvateils, and one lashed out. The renegade Reyvateil Mir struck out in a genocidal campaign to make a utopia for Reyvateils and none other, causing a civil war and destroying much of the First Tower's systems in the process before she was sealed away. And in the age to follow, even much of that was lost to the annals of history and legend.
Now, after Mir's revival and the near destruction of the tower at her hands once more, she was defeated by a young knight and his band of companions and Reyvateils, given a second chance to help them build a new era of peace.