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.

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.

Naofumi has obtained many abilities and skills, and his teamwork with Raphtalia is improving as well. To prepare for the incoming Wave, he upgrades their equipment and heads to the dragon hourglass to find out exactly when it will arrive.

The Wave is gone, but Naofumi's relief is short-lived as Motoyasu challenges him to a duel. Motoyasu insists that owning a slave is unbecoming for a hero and that Naofumi free Raphtalia at once.

After Aultcray reluctantly offers Naofumi a paltry reward, he heads to the slave merchant to redo the slave crest as per Raphtalia's wishes. He also buys a monster egg there, which hatches a few days later and gives him a new party member.

The mysterious girl that showed up out of nowhere was actually Filo. She's a filolial queen with an advanced transformation ability, and she can even take human form thanks to it.