Although it was already predicted by his party member Cassandra Illion, a terrible disaster strikes upon Bell Cranel as he falls into the depths of the Dungeon. Wounded, lacking equipment, and secluded from the rest of his party, Bell can only rely on Ryuu Lion, who is in the same situation. Pursued by a monster whose power far exceeds both their skills, the only hope the improvised duo of adventurers has is to safely make it to a monster-free point even deeper in the labyrinth.

Meanwhile, the rest of Bell's party must fight the floor boss that has unexplainably respawned ahead of schedule. As Bell and his friends grapple for survival, they discover that the cruelty of the Dungeon is more boundless than ever.