While enjoying an ordinary day in town, Mari suddenly finds herself surrounded by people collapsing, and some transforming into monsters. Guided by a mysterious voice, she is given the ability to fight back against the evil creatures.

It's another ordinary day in the ordinary lives of Mari and Kotone when something extraordinary happens: the townspeople begin to collapse one after another before their very eyes and transform into horrific monsters! Somehow unaffected by the unknown phenomena, Mari is guided by an ethereal voice and gains the ability to transform into Lilium, a heroine with the power to face the monsters head-on and send them packing. However, saving her home won't be as simple as clobbering her former friends and neighbors... The reality is that the door to another world has opened, and miasma is pouring out through the cracks, turning the town she knows and loves into an unrecognizable hellscape. Only by venturing into the unknown and staging an invasion of her own can Mari hope to stymie the threat and protect all she holds dear.

And yet, there are still so many questions left unanswered. What's the truth behind the miasma and the terrifying monsters that roam the streets? How and why did the two worlds become connected in the first place? And perhaps most importantly, is there truly a way for Mari to reverse the damage that's already been done and reclaim her old life?