The orphans Mary and Metea want to join Nao's party as adventurers, so Nao and his friends decide to bring them along and explore a dungeon so that they can gain first-hand experience. However, although it's their first time embarking on such an adventure, the young sisters display an unexpected form of strength. The dungeon that they explore is a strange and mysterious one in which the environment changes with each floor. The bond between the members of the Meikyo Shisui party, both old and new, will be put to the test!