Ar Tonelico is a JRPG series about a world in the future after humanity has managed to nearly destroy itself twice and is thus confined to living on and around massive towers that appear to float above the surface of the ruined world. The world is populated by a fraction of the current population, who live in scattered small towns. The cast thus consists of the knights, adventurers and Reyvateils who attempt to keep the world from being ruined yet again.
Ciel nosurge: Ushinawareta Hoshi e Sasagu Uta and Ar nosurge: Ode to an Unborn Star are a pair of games forming Gust Corporation's Surge Concerto series. This series serves as a distant prequel to Gust's Ar Tonelico games, though direct connections are not evident right away.
Together, the two series make up the EXA_PICO universe.