Though he has successfully repelled the latest Wave of Catastrophe, Naofumi Iwatani—the Shield Hero—has no time to rest. Naofumi is spurred back into action when Queen Mirelia Q Melromarc tells him that the three other Cardinal Heroes have disappeared without a trace, and she tasks him with finding them.

With no clues regarding their whereabouts, Naofumi tackles a more pressing issue: the worsening slave trade of demi-humans. Determined to right this injustice, he and his friends find their way to Zeltoble, the country of mercenaries, where the illegal sale of demi-humans has become a lucrative business. To gather money to free the slaves, Naofumi and his comrades disguise themselves and take part in underground coliseum brawls, purposely throwing matches to manipulate the odds and eventually secure an enormous payout. However, with the threat of another Wave of Catastrophe looming on the horizon, the Shield Hero must stick to his mission of finding the missing heroes if he wants to protect those he loves.