In 2021, a parasitic virus called "Gastrea" attacks humanity and transforms them into monsters. What remains of humanity now lives inside the Monolith walls, which are composed of Varanium, the only substance capable of harming Gastrea. To address the threat that the Gastrea offer, "Cursed Youngsters"—female children whose bodies carry trace levels of the virus that grants them superhuman abilities—are assigned Promoters, persons who work to guide and safeguard the young Initiators by the Tendo Civil Security. These two-person teams are dispatched on missions to combat the Gastrea virus's monsters and keep them at bay. The plot of Black Bullet centres around the duo of Enju Aihara, an Initiator, and Satomi Rentaro, a Protector.