Black Bullet, Vol. 2 (manga) (Black Bullet (manga), Band 2)

The monstrous mutants created by the Gastrea virus have driven humanity to the brink of extinction. The only thing keeping them out of Tokyo are the giant Monoliths that surround the city and the efforts of civil security officers like Rentaro and Enju. But when they accept a strange mission from the Ministry of Defense, they uncover a plot by Kagetane Hiruko that could end in the destruction of humanity!