Life in the bustling city of Orario is never dull, especially for Bell Cranel, a naïve young man who hopes to become the greatest adventurer in the land. After a chance encounter with the lonely goddess, Hestia, his dreams become a little closer to reality. With her support, Bell embarks on a fantastic quest as he ventures deep within the city's monster-filled catacombs, known only as the Dungeon. Death lurks around every corner in the cavernous depths of this terrifying labyrinth, and a mysterious power moves amidst the shadows. Even on the surface, survival is a hard-earned privilege. Indeed, nothing is ever certain in a world where gods and humans live and work together, especially when they often struggle to get along. One thing is for sure, though: a myriad of blunders, triumphs and friendships awaits the dauntlessly optimistic protagonist of this herculean tale.

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.

It is business as usual in the massive city of Orario, where legions of adventurers gather to explore the monster-infested Dungeon. Among them is the easily flustered yet brave Bell Cranel, the sole member of the Hestia Familia. With the help of his demi-human supporter Liliruca Arde and competent blacksmith Welf Crozzo, Bell has earned the title of Little Rookie by becoming Orario's fastest-growing adventurer thanks to his endeavors within the deeper levels of the Dungeon. Dungeon ni Deai wo Motomeru no wa Machigatteiru Darou ka II continues Bell's adventures as he tries to bring glory to his goddess and protect those he cares about. However, various familias and gods across the city begin to take notice of his achievements and attempt to add him to their ranks.

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.