The destruction of the world by fire

Rentaro, Enju, Kisara, and the fearsome sniper Tina--just when they thought the future was looking slightly less uncertain, they're confronted with a terrible discovery: One of the giant monoliths that protects Tokyo Area from invasion by the monstrous Gastrea is on the verge of collapse, putting the lives of every single human within the city at risk!

Is there any way to save Tokyo Area? And even if there is, what will become of Rentaro, Enju, Kisara, and Tina?