Centuries after nuclear annihilation reduced civilization to underground bunkers, humanity clings to survival in fortified shelters scattered across a devastated world. In 2036, two companions whiling away their time in Keihin Shelter No. 3 with a video game called Devil Busters accidentally unleash Pazuzu, a demon who grants them access to a forbidden Demon Summoning Program—a tool meant to help them combat the greater threat of Bael and ward off an approaching calamity. As demonic forces begin breaching the shelter's defenses and threatening the last remnants of human civilization, the unlikely pair must master their newfound power to summon and command creatures from the supernatural realm to defend their underground home from obliteration.