The game takes place in the near future, where warped monsters and ghouls have appeared from nowhere, swarming cities and forcing humans to battle against these creatures they've termed "demons" in an all-out Demon War. After 30 merciless years of war, the surviving humans have banded together under the Missa Association, a religious group formed by a leader who knows the secret to fighting the demons. Cities and civilizations continue to thrive despite the now regular sightings of bizarre monsters.
Three years ago, a man known as Shin was rescued from among the rubble in a cryopreserved state. With no recollection of his past, he quickly becomes one of the association's best warriors and protectors.
However, when Shin comes across the brilliantly beautiful demon named Lux, she feels strangely familiar. This sensation of deja vu leads him to take her in, even if it means going against the creeds of the association.
From that very instant, the past begins to slowly unfold, revealing the events that caused the city to take on its current form...