Based on COLOPL's school action role-playing game. Set in the year 2045. The world has been contaminated by Irousu (mysterious invaders who suddenly appeared), and humans find themselves restricted and contained. Standing boldly against these invaders are ordinary girls everywhere, without a powerful army or even weapons. The Shinjugamine Girls Academy is a school for these "Hoshimori" (Star Guardians) destined to fight the Irousu. The player is assigned to this academy to train the girls and take back the contaminated Earth. And so, once again, the chimes echo through the sun-strewn schoolyard to mark the beginning of classes today...

After having descended upon this world, the gods have created guilds where adventurers can test their mettle. These guilds, known as "familia," grant adventurers the chance to explore, gather, hunt, or simply enjoy themselves. Dungeon ni Deai wo Motomeru no wa Machigatteiru Darou ka Gaiden: Sword Oratoria begins in Orario, the lively city of adventures. The Sword Princess, Ais Wallenstein, and the novice mage, Lefiya Viridis, are members of the Loki Familia, who are experts at monster hunting. With the rest of their group, they journey to the tower of Babel in hopes of exploring the dungeon underneath. Home to powerful monsters, the dungeon will fulfill Ais's desire to master her sword skills, while bringing Lefiya closer to her dream of succeeding Riveria Ljos Alf, vice-captain of the Loki Familia, as the most powerful mage in the land.

Continuing his adventure to get stronger in order to traverse deeper into the "Dungeon," Bell Cranel wanders the Orario city streets with his friends and the goddess Hestia. That evening, the city is filled with stalls and games as it celebrates the Holy Moon Festival.

Hermes, a god, hosts one such activity where participants are asked to pull a spear embedded in a crystal boulder; those who succeed will receive a special gift: a trip around the world and a divine blessing from the gods! Bell and his merry group challenge one another to claim the prize. But behind the facade of an innocent party game lies a preface for a daring quest ahead.

The anime's story will start where the third season left off, at the competition for who will open the Triple S international sporting event. Raging Entertainment's HE★VENS idol group had hijacked the competition between QUARTET NIGHT and ST☆RISH, causing chaos. However, after the QUARTET NIGHT members suggest a certain idea, the three groups aim toward a new stage, with HE★VENS swearing to get their revenge on the other two groups.