The psychologically destitute Kuro Fukase has been betrayed by the cruel world countless times. The only people who he could ever call family were Toiro and Iroha, the two Ayakura sisters living in an adjacent apartment, but even they would soon perish in a horrific act of arson. Now, the siblings exist as mere hallucinations in Kuro's mind as he continues to bear hatred for the world.

On one bitter Christmas Eve, Fukase's very purpose in life changes. Upon meeting the strange yet cheerful girl Sera Himeura, the dubious and alluring witch Yume Furusuna, and a strange black creature, Fukase learns that he is one of 20 "Instant Bullets": people who have been granted the power to destroy the world.

However, these living weapons do not share the same goal. Some wish to save the world, while others—including Fukase—are hell-bent on its obliteration. In a suspenseful tale of past trauma, faltering morals, and twisted romance, just what future does the world hold when the powerful Instant Bullets turn against each other?