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 foils the attempt to take Melty's life and fights off the royal knights, but he's then framed for abducting Melty and killing several knights, so the party is forced to go on the run. With security tightened and checkpoints everywhere, the party receives a further blow that only makes things even worse.

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?

After Naofumi survives thanks to Mirelia's help, she comes to visit him and tells him the truth behind the summoning of the heroes and the discrimination against the Shield Hero. She also promises to clear his name, and holds a trial for Aultcray and Myne's crimes.

After being unable to for so long, Naofumi finally gets to upgrade his party's classes. He then takes part in a meeting of the Four Cardinal Heroes, held under the guise of a celebratory banquet. There, he learns of the existence of a bonus stage, with increased XP earnings. Meanwhile, Raphtalia gets into a fight at the party.

On the ship to Cal Mira, Naofumi once again runs into L'arc, who keeps throwing him off. However, the strange man's friendly nature shines through, and Naofumi quickly takes a liking to him. After having a fun time grinding levels and a lively meal, Naofumi feels happy and fulfilled for the first time in this world.

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...

It turns out that Glass, like L'arc, is also a hero from other world. Naofumi barely fights off their combo attacks and then corners Glass in a one-on-one battle, but is then faced with an all-important question: does he truly wish to protect this world?

Mysterious monsters appear as Naofumi is preparing for the next Wave. At the same time they show up, the Wave countdown stops. The Queen summons the Four Cardinal Heroes, and tells them that the Spirit Tortoise has revived.

The leaders of all the nations’ militaries gather in the Spirit Tortoise Kingdom. Naofumi is there as well, and promises to offer his help as one of the heroes. But just before they decide on a plan, a mysterious woman appears…