College student Iwatani Naofumi is reading a book titled "The Records of the Four Cardinal Weapons" in the library when he is suddenly summoned to another world and told he has to save the world as the Shield Hero, one of the Four Cardinal Heroes. And one morning, he finds all his money and equipment gone.

Naofumi follows the slave merchant to his store, where he looks for a partner to act as his sword. But all he has the money to buy is a diseased demi-human girl named Raphtalia, whom he then forces into an incredibly harsh battle.

The Wave is gone, but Naofumi's relief is short-lived as Motoyasu challenges him to a duel in the middle of the celebratory banquet held in Melromarc Castle. Motoyasu insists that owning a slave is unbecoming for a hero and that Naofumi free Raphtalia at once.

After hearing of a village that's in immediate need of herbicide, Naofumi and his party head there to do business. The village has been overrun by plants that have turned into monsters, and its residents were forced to abandon it. These plants can even take root in humans, and apparently Motoyasu is the one who released them into the wild.

Naofumi faces another Wave, this time in a remote village. With the help of the soldiers who volunteered to join him, he's able to secure the safety of the villagers, but the Wave shows no signs of letting up. He heads to fight with the other heroes, only to find a giant ghost ship floating in the sky.

A mysterious woman showed up out of nowhere and destroyed the second Soul Eater in a single blow. She then proclaimed the start of the "true battle of the wave" and blew away the other three heroes when they attacked her. With Naofumi's party proving ineffective as well, he decided to use the Rage Shield one more time.

Due to Myne recklessly starting a forest fire, Naofumi and his party are forced to flee to a village at the base of the mountain. However, it won't be easy to break through the knights' net. Melty suggests going to a noble she knows for help, and it just so happens that Raphtalia's past is linked to his domain.

As Naofumi and his party try to get the weakened kids to safety, Tyrant attacks them. Once Naofumi realizes that he's actually after Filo, he hatches a plan to lure him outside the city, but they can't even put a scratch on its massive body.

Naofumi decides to go straight up to the checkpost in order to talk it out with the other heroes. But there, he finds the one guy least likely to listen to him, Motoyasu, who attacks Naofumi without bothering to listen to what he has to say. Why could Motoyasu be so intent on killing Naofumi, more than ever before?

Naofumi and gang stay on the attack despite being trapped in the Pope's Great Cathedral, but its sacred subspace nullifies his Curse Shield, leaving them one step short. Naofumi turns to another, newly unlocked Curse Shield, but the dragon's wrath threatens to swallow his mind whole.

Melromarc brings a large fleet to fight the Wave out at sea. While the queen takes charge and leads the operation well, the other three heroes prove useless yet again, leaving Naofumi to do it all alone. Later, he gets help from L'arc and Therese to fend off the monsters, but then...