The Four Cardinal Heroes are a group of regular guys from present-day Japan called to the kingdom of Melromarc to serve as its saviors. Melromarc is a country suffering from the Waves of Catastrophe, which have repeatedly devastated the land and brought disaster to its citizens for centuries. The four heroes are each given a sword, spear, bow, and shield to defeat these Waves. Naofumi Iwatani, an otaku, is cursed with the role of the "Shield Hero." With nothing but a weak shield, Naofumi is looked down upon and mocked by the other heroes and the kingdom's citizens because of his poor offensive power and unimpressive personality.
After the heroes receive supplies and companions to train alongside, Naofumi begins his journey with the sole person who agrees to join him, Malty Melromarc. However, she soon betrays him, and he is wrongly accused of assaulting her. As a result, Naofumi faces severe discrimination and hatred from the people of Melromarc for a crime he never committed. With deep hurt and distrust burning inside him, Naofumi sets out to build his own strength and repair his reputation. But as he goes on, the hardship of traveling alone becomes too great, so Naofumi purchases a dying demi-human slave named Raphtalia to join him.
As the Waves draw closer to the kingdom, Naofumi and Raphtalia must battle to ensure the kingdom's survival and shield its people from their doomed destiny.