A young chemist girl tackled the once-unbeatable Labyrinth Farwitch alone, in order to save her nearby village from being a casualty to the awakening of the Demon Lord. She surprisingly succeeded, but never returned, leaving behind a precious childhood friend and her young sister.

Five years after, local librarian Key Falen is invited by the newly-summoned hero and his cohorts to conquer the Labyrinth in order to solve the mystery of the Demon Lord's defeat.

The mystery behind his childhood friend grows larger the more Key learns about her...