It is a continuation of Ciel nosurge released in 2012, and also a prequel to the Ar tonelico series.
The story of the game is told through the viewpoints of two character pairs: Cas & Delta, and Earthes and Ion. Initially, Cas and Delta are tasked to find out a song called Ciel N Protecta that their friend Sarly developed, but it was stolen and it supposedly has begun causing the machines in Felion to go haywire. During their searches for it, they are captured by the Genomirai Church led by Zir, and as they try to escape from them, they discover Zir is planning to set up and sing a very dangerous kind of Song Magic. On the other hand, Ion is trapped in the sealed world through which the player interacted with her in Ciel nosurge, and having Earthes as her helper, she decides to escape from it. After escaping from the sealed world, they both reach the city of Tenryosara, where they meet with Tattoria and Kanon. And from there, both pairs continue their journey in search of the truth about this world.
Ar nosurge Plus is an enhanced remake of the game for the PlayStation Vita platform. Additional content added to the game include a greater number of costumes, touchscreen interaction compatability for purification ceremonies, over 50 new events and additional characters available for purification ceremony, a new tutorial mode, and all post-release DLC from the original PlayStation 3 version.
Source: Wikipedia, "Ar nosurge," available under the CC-BY-SA License.