A sudden SOS arrives from Riselia's birthplace, the Third Assault Garden, which was supposed to have been destroyed six years ago. To ascertain the truth behind it, Leonis and the others head for the abandoned city haunted by Voids! What awaits them at this nightmare from the past?!

A sudden SOS arrives from Riselia's birthplace, the Third Assault Garden, which was supposed to have been destroyed six years ago. To ascertain the truth behind it, Leonis and the others head for the abandoned city haunted by Voids! What awaits them at this nightmare from the past?!

After the dust settles from the Hyperion incident, Excalibur Academy receives a strange distress signal--one that appears to be from the Third Assault Garden. The problem? The Third Assault Garden should have been destroyed by Voids six years ago! Determined to find out what happened to her birthplace, Riselia leads the eighteenth platoon on a new mission: exploring the abandoned city. But what awaits them may be more than just ghosts from the past...