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.

Adventuring knight Laios Touden leads a small party through a seemingly endless dungeon, a subterranean maze full of dangerous monsters and precarious traps. Through the use of advanced magic, an explorer can sometimes be resurrected, allowing them to learn from past mistakes and give traversing the dungeon another go. However, when a powerful dragon eats Falin, Laios' spellcasting sister, she sends her brother and his companions back to the beginning to save them from permanent ends. Though strapped for cash and equipment, Laios resolves to fight his way through the dungeon and rescue Falin before she can be digested by the dragon. Despite some of Laios' allies abandoning him, two remain by his side: elven mage Marcille Donato and halfling locksmith Chilchuck Tims. Due to their lack of funds, the party faces the daunting prospect of starving before being able to complete their quest. However, they find salvation in Senshi, a dwarven warrior with a penchant for cooking and safely eating defeated monsters. With Senshi's culinary expertise, Laios and his companions struggle through the dungeon while learning about gourmet dining—and each other—along the way.

Fifteen years have passed since the war between humans and demons began. Dissatisfied with their slow advance into the Demon Realm, the Hero abandons his companions to quickly forge ahead towards the Demon Queen's castle. Upon his arrival at the royal abode, the Hero makes a startling discovery: not only is the Demon Queen a woman of unparalleled beauty, but she also seeks the Hero's help. Confused by this unexpected turn of events, the Hero refuses to ally himself with his enemy, claiming that the war the demons have waged is tearing the Southern Nations apart. However, the Demon Queen rebuts, arguing that the war has not only united humanity but has also brought them wealth and prosperity, providing evidence to support her claims. Furthermore, she explains that if the war were to end, the supplies sent by the Central Nations in aid to the Southern Nations would cease, leaving hundreds of thousands to starve. Fortunately, she offers the Hero a way to end the war while bringing hope not only to the Southern Nations, but also to the rest of the world, though she will need his assistance to make this a reality. Finally convinced, the Hero agrees to join his now former enemy in her quest. Vowing to stay together through sickness and health, they set off for the human world.

Following the First-Class Mage Exam, the trio—elven mage Frieren, warrior Stark, and first-class mage Fern—gains access to the dangerous Northern Plateau. As the party presses onward toward Aureole, formidable adversaries force Stark to confront his insecurities, solidifying his resolve and his role as the party's frontliner. Meanwhile, Fern continues to cherish the gifts she has been blessed with throughout her life, each a reminder of those she holds dear. Frieren—still honoring her vow to understand humanity—revisits memories of her journey with the Hero's party and her fleeting encounter with a legendary figure. As she reflects on the passage of time, the elven mage quietly questions whether she has truly changed, yet in the small, almost subtle choices that she makes, there are signs that she might have become more human than she realizes.