It's been 150 years since Humans left Earth. 150 years since the Guardians recruited them into the UCOM military to help fight the Slayers. 150 years to prepare for war.
An unidentified spacecraft enters the Milky Way. Humanity's worst fear is realized. The Slayers have arrived.
The Milky Way is ravaged by war. A sect of Human religious zealots exploit this window of opportunity to side with the enemy - who they consider to be Gods, coming to bring humanity to salvation. Blinded by faith, the Destineers cripple UCOM defenses as they aide the Slayer invasion. Humanity is cast into battle as they rise to meet a superior enemy.
They fight for their galaxy. They fight for Earth.
They fight for life.