Wild Arms: the 4th Detonator

Jude Maverick has grown up in an isolated town called Ciel, which is completely enclosed in a large sphere floating thousands of feet above the surface of Filgaia. His sheltered world changes forever when he sees the sky "tear" and ships enter his homeland. Upon inspecting the campsite set up by the intruders, he sees something else that he has never seen before: a girl.
The girl's name is Yulie and she is being held captive by the intruders after being captured by a "Drifter" for hire, Arnaud G. Vasquez. After the invading army attacks Ciel and brings the sphere crashing down, Jude, Yulie and Arnaud join forces to find the army's true intention behind Yulie's abduction. With the help of another Drifter, Raquel, these four idealistic teens travel the war-torn land of Filgaia in search of truth, their own identities, and their separate paths to adulthood.