From DrCoke at MangaHelpers:

Eight years ago, a girl went missing while playing hide-and-seek in the forest. Starting around then, instances of 'labyrinths' started spawning in random places, which led to an increasing amount of missing people. Kaori was one of them, when she was never found after the game of hide-and-seek she played with Shuhei. Now, eight years later, Shuhei still feels guilty for the loss of Kaori. He feels as though he, too, should be dragged into these 'labyrinths' in the search for his long lost friend. Now, he found an opening into the labyrinth. In it, he begins his search for the truth behind this phenomenon!