The Game:
At the end of Golden Sun, two of the four elemental lighthouses have been lit. Even though Isaac's party has destroyed their enemies, his childhood friend Jenna is missing, and her brother Felix seems to be working against them. The Lost Age picks up at the same place that story left off, only now it continues from Felix's perspective. For both personal and heroic reasons, the lighthouses must be lit, and there's a long journey ahead to get to them. The quest leads to new allies, new lands, and finally a rejoining with Isaac's group that allows control of all eight heroes. When the full impact of the lighthouses is understood, they all need to work together to finish the relighting. With the new territory also comes more Djinni, more challenging sidequests, new classes, and new psyenergy to complete your tasks.

This is only a vague outline of the story, so as not to spoil anything. For more information and walkthroughs, try the following sites:
Official Site
Golden Sun Realm

The Fanlistings:
TFL.org is a network of thousands of sites that simply list fans of thousands of different subjects. The idea was to unite fans in a simple and direct way, to show support for whatever you're a fan of. This is the only GS:TLA fanlisting approved by TFL.org.