The game picks up 30 years after the first two games in the series left off. At the end of Golden Sun: The Lost Age, the heroes succeeded in bringing the world-healing power of the Golden Sun back to the world of Weyard. In the years since, the immense power of the Golden Sun has changed nearly everything. Continents have shifted. New countries have emerged. New species have appeared. But the world is now imperiled by a new threat. Psynergy Vortices, which suck the elemental Psynergy from both the land and power-wielding Adepts alike, are spreading across the world. The new generation of heroes - the children of those from the previous games - are drawn into the mystery behind the vortices as they cross a chaotic world that is succumbing to a new evil.
Source: Wikipedia, "Golden Sun: Dark Dawn", available under the CC-BY-SA License.