The Septentriones, alien invaders bent on eradicating humanity, arrive on Sunday, starting the countdown to extinction. Caught in the crossfire, Hibiki Kuze and his pals join the struggle for humanity's survival by signing contracts with demons to become "Devil Summoners," attracting the notice of JP's, an underground organization run by Yamato Houtsuin. Hibiki and his buddies fight and connect with other regular individuals who are Devil Summoners after being recruited into JP's. With each new day, though, another Septentrione arrives to wreak devastation on Japan. Even if many lives are sacrificed in the process, the youthful summoners must destroy the invaders at all means before the night is through.