In A.G. 101 (the 101st year of the Advanced Generation calendar) a mysterious entity known only as “UE”, or “unknown enemy”, attacks and destroys the space colony Angel. This brutal attack becomes infamous as the “The Day the Angel Fell”, and marks the beginning of humanity’s war for survival. The series begins in A.G. 108 when the UE attack the space colony Ovan, where Flit Asuno lives with his mother.