The story is split into two stages, two different worlds. On one side is an ordinary town very similar to ours. Our heroine, Mari, lives a very everyday life as a student along with her best friend, Kotone.
On an ordinary day, Mari would head to school, enjoy her club activities, and then head into town and eat some sweets, just as always. However, what awaited them in town was unexpected. Suddenly, everyone around the two began to collapse. Some of them turned into monsters and began attacking them.
The only unaffected Mari was guided by an unknown voice and changed into a mysterious outfit. Completely unsure of what was happening around her, she had no choice but to use the weapons she possessed and defeat the monsters.
Mari manages to save Kotone and is summoned to a certain facility. Some men were awaiting her and explained what was going on. Another world existed, and from it, miasma and monsters were coming into hers. They also informed her that because she was one of the very few humans with natural resistance to the miasma, she had the capabilities to be able to take on those monsters head-on.
Mari learned that the monsters were still trying to flood her world and had to be defeated in the other world to make everything that had happened to her world nothing more than a dream. To achieve that, she went off as the transforming heroine, Lilium, to the new world, determined to stop the monsters.