Bell Cranel's rapid growth, his victory in the war games, and the increase in his family members, among other things, has lead to a continuing rise in the visibility of the Hestia Familia in the labyrinth town, Orario. With advances in the dungeon going well, they have arrived on the Nineteenth Floor - the labyrinth of the Great Tree. In the height of battle, Bell is forced to act solo and rescue a girl being attacked by monsters... But this trembling, crying girl is, herself, a monster.

After her form was exposed in town, causing an uproar, Bell and the others managed to rescue Weine. As they confront the difficulty of sheltering her aboveground, the Guild charges the Hestia Familia with a compulsory mission - to go to the dungeon's Twentieth Floor and take Weine with them. Unable to figure out what the Guild is trying to do, and with no choice but to comply, Bell and the others head back down into the dungeon...