After a touching mother-daughter reunion in the city of Erisen, Hajime and his party resolve to take on the fourth Great Labyrinth, the underwater ruins of Melusine. Every labyrinth presents unique dangers, and this one summons up visions of the past to test those who dare to enter. Separated from the rest of their comrades, Kaori and Hajime will be challenged like never before. Will their relationship change in the fires of hardship?