The world is facing a disease of unknown origin, resulting in a large number of humans going missing or mutating into monsters that only know killing and eating. The survivors have formed various resistance and self-defense organizations to fight against these monsters in order to survive.

Control the protagonist and use bullet hell attacks to eliminate enemies. During battles, the protagonist can acquire and upgrade various additional weapons to enhance combat abilities. 

You continue fighting until your stamina runs out (HP reaches zero), then return to the base to upgrade your abilities before heading back into battle.