University student Naofumi Iwatani finds himself summoned to the kingdom of Melromarc, a fantasy world threatened by periodic catastrophes known as the Waves of Catastrophe. He arrives as one of four Cardinal Heroes, each wielding a legendary weapon: the Sword, the Spear, the Bow, and Naofumi’s Shield. The kingdom's people revere the Sword, Spear, and Bow heroes, but the Shield Hero is met with disdain and prejudice due to a religious doctrine that venerates the other weapons while vilifying the shield.

The situation turns catastrophic for Naofumi on his second day. His sole party member, the princess Malty Melromarc, steals his money and falsely accuses him of rape. The other heroes—sword-wielder Ren Amaki, spear-user Motoyasu Kitamura, and bow-fighter Itsuki Kawasumi—believe the accusation, and King Aultcray publicly denounces Naofumi as a criminal. Stripped of his reputation, funds, and any support from the kingdom, the bitter and untrusting Naofumi is forced to survive on his own.

Left with no other options, Naofumi purchases a sickly and frightened demi-human slave girl named Raphtalia from a merchant. As they battle monsters and level up, Raphtalia grows rapidly in both strength and age due to a demi-human trait that links physical maturity to level progression. She becomes a skilled and beautiful swordswoman, and her unwavering loyalty gradually begins to heal Naofumi’s psychological wounds, transforming their transactional master-slave relationship into a genuine, trusted partnership. They are soon joined by Filo, a small bird-like monster that hatches from an egg and unexpectedly evolves into a powerful, humanoid filolial queen capable of pulling a carriage and fighting with devastating kicks.

As Naofumi builds his small party, the other three Cardinal Heroes treat their roles like a video game, acting recklessly and causing disasters across the countryside that Naofumi is left to clean up. Motoyasu releases a dangerous plant that destroys a village, Ren leaves a dragon corpse to rot, causing a plague, and Itsuki’s misguided justice leaves a power vacuum that worsens the lives of common people. Naofumi, meanwhile, unlocks dangerous new abilities from his shield, including the Curse Shield powered by his own wrath, which offers immense power at the risk of consuming his sanity.

The narrative reaches a turning point when Naofumi is framed for the abduction of the young princess Melty, the kind and intelligent younger sister of his accuser Malty. Pursued by the kingdom’s knights, Naofumi's party fights to clear his name. The conspiracy deepens when they clash with the corrupt Church of the Three Heroes, which seeks to eliminate the Shield Hero entirely. With the help of the returning Queen Mirellia, the true ruler of Melromarc, Naofumi's name is finally cleared in a public trial. The queen sentences her husband, King Aultcray, and her daughter Malty for their treasonous collusion with the church, though Naofumi halts their execution, instead insisting they be renamed Trash and Bitch as a permanent humiliation.

After this vindication, the story shifts to training for the next Wave at the Cal Mira Archipelago, a resort island where experience points are greatly amplified once a decade. While leveling up, Naofumi befriends two powerful adventurers, L'Arc and Therese. However, when the next Wave occurs at sea, these allies reveal their true nature: they are heroes from another world also ravaged by the Waves, and to save their own dimension, they must destroy the heroes of Naofumi’s world. They are joined by Glass, a mysterious and powerful fighter from a previous Wave. The season concludes with the revelation that the battles between worlds are just beginning, and Naofumi must now prepare to face not only monsters but also heroes from another realm determined to see him fall.