A young girl is walking her dog Poro around town at dusk when a tragic accident forces her to return home with only Poro'sleash in hand. The girl's sister commands her to stay home while she goes out into the night to search for the dog. Time passes. Growing impatient, the girl decides to search for her missing sister and dog herself.
But when she steps foot outside, she realizes that this is not the same town. The darkness has made the town unfamiliar, and she appears to be utterly alone...save for the spirits that stalk her. The girl must hide from, outsmart, or outrun the hunting spirits while she searches for clues to find her loved ones.
In the depths of a labyrinthine ruin, a young girl named Mion rises from a deep sleep. Lost, alone, and with no memory of how she got here or where her parents are, she is greeted by two fireflies—one composed entirely of light, and another who lurks among the shadows.
By shifting between a world of light and a world of shadows, these two fireflies must work together to recover Mion's memories and lead her out of the ruins, where voracious shadow beasts, ancient machines, and treacherous traps lie in wait to bar her path. But what secrets do Mion's lost memories contain? What is the connection between her, the ruins, and the fireflies lighting her escape? The answers may reveal a deeper darkness—one that must be faced from within...