Adult PC Game: Hasoyua - Egg of World - Illusory Lilium Ver.2.03 Final (Official Translation) Archive

Two different worlds...
One is very much like our own. It was there that schoolgirl Mari and her best friend Kotone lived their lives in peace.
On that fateful day, they went to school, did their extracurriculars, and went to hang out at a cafe afterward.
At least, that's what was supposed to happen...

Instead, they were greeted by the sight of citizens collapsed on the ground, some of whom turned into raging monsters.
Mari, who seemed immune to the evil influence, called forth a mysterious costume. She had no idea how, but was able to defeat the creatures with her mysterious new weapon.
She'd saved her friend Kotone, and was then summoned to the headquarters of a strange organization. There, she was first informed of the existence of another world... and of the evil miasma and fearsome monsters that dwell within.
And thanks to her immunity to the otherwordly vapors, it was up to her to face the threat head-on!

The monsters mean to invade the town even now, but if the threat can be contained early, it will be as if it never existed in the first place.
And so, in order to fulfill her inescapable duty, Mari transforms into her alter-ego Lilium and sets off to the other world, hoping to stop the monstrous miasma at its source.