Twenty-year-old otaku Naofumi Iwatani is mysteriously transported to the otherworldly kingdom of Melromarc. Appearing before the King, Naofumi and three other summoned individuals learn that they are each one of the Four Cardinal Heroes who are tasked with defeating the malicious "Waves of Catastrophe."

However, Naofumi is given only a measly shield and dubbed the "Shield Hero," the weakest of the Four Cardinal Heroes. Owing to his lack of power, none of the kingdom's mercenaries wish to aid him, except for the country's beautiful princess, Malty Melromarc. But Naofumi soon awakens the next day to find himself robbed of all his possessions and falsely accused of rape by Malty.

Due to his status as a Cardinal Hero, Naofumi is not executed, but instead, his erroneous crimes are made public. Struggling to find hospitality, Naofumi buys a demi-human slave girl named Raphtalia to aid his endeavors, and embarks on a perilous journey to deliver retribution to those who ruined him, learning the secrets of this new world along the way.