Summoned to the kingdom of Melromarc, four men from modern-day Japan are designated as the saviors against the recurring Waves of Catastrophe. Naofumi Iwatani, an ordinary otaku, is chosen as the 'Shield Hero' and is equipped solely with a shield, earning scorn from his comrades and the people of Melromarc for his defensive role. Betrayed and falsely accused, Naofumi faces discrimination and animosity, leading him to toughen himself with the help of a demi-human slave, Raphtalia. Together, they must combat the Waves and defend the kingdom against impending doom, battling not only external threats but also the internal turmoil within Naofumi's heart.