After saving the Xenos from a mortal peril, adventurer Bell Cranel finally achieves Level 4. Now strong enough to head toward the dangerous depths of the Dungeon, the captain of Hestia Familia embarks on a new adventure with his friends and allies.

However, according to a prophetic dream of Cassandra Ilion, a looming catastrophe caused by Bell's close friend Ryuu Lion risks causing the demise of the entire party. Nevertheless, Bell is unwilling to distrust Ryuu and determined to clear her name—but only time will tell if he is going to be strong enough to save her and prevail against the cruelty of the abyss.

The series adapts the volumes 12 and 13 of the light novel of Fujino Omori's series of the same title.