From MangaHelpers:

10 years in the past, the city of Tokyo was consumed by an unknown darkness. It started off as a black speck in the sky. As it grew in size, it consumed everything in its path, creating the area known as the "Lightless Zone."

Now, from the darkness arises beings known as "Imashime" ("The Warning"), attacking people during the night. Humanity's only weapon against these Imashime are the "Vess," fighting machines equipped with a piercing weapon that can stop the Imashime from regenerating itself. Saginuma Akashi is at the top of his class when it comes to piloting the Vess. Now, he and a select other few have been selected to be the first battalion to invade the "Lightless Zone" for the first time. [DrCoke]