You’re an easygoing Japanese youth, was summoned into a parallel world along with three other young Japanese men from parallel universes to become the world's Cardinal Heroes and fight inter-dimensional hordes of monsters called Waves. Each of the heroes were respectively equipped with their own legendary equipment when summoned. You happened to receive the Legendary Shield, the sole defensive equipment, while the other heroes received respectively a sword, a spear, and a bow, weapons meant for attack. Unlike the other heroes who are fully supported by the kingdom and gain several strong allies each, your luck turns to the worse after your single companion, revealed to be the kingdom's princess, betrays you, steals all your belongings, and leaves you devoid of all assistance and supplies after she falsely accuses you of sexually assaulting her.

Mocked by the nobility and shunned by everyone from your fellow heroes to peasants, a now cynical you is forced to train as a hero alone while working to make ends meet, until you buy from a slave trader a young tanuki demi-human girl named Raphtalia and an egg that hatches into a bird-like monster who you named "Filo", both quickly growing into adulthood and becoming powerful warriors under your care. As they little by little gain the trust and gratitude of the people with their heroic actions, you and your companions work together to carry out their mission as saviors as they unravel the mystery of the Waves and the reason why they are a threat not only to their world, but to other worlds as well.