Exploring a new empire! Allen and the No-life Gamers delve deeper than humanity ever was capable: to the Prostia Empire on the ocean floor! In search of Holy Orbs to buff his party and hoping to broker peace between Crevelle and Prostia, Allen takes the form of a merfolk and acts as an ambassador between the two nations...or so he wants everyone to think. In truth, he wants to catch Beast Crown Prince Beku, the man who revolted against his nation to claim the throne only to flee when he failed. Are the rumors of the merfolk's involvement in the rebellion true?

Upon reaching Prostia, however, Allen finds that nothing but problems lurk beneath the water's surface. All is not well with the once-mighty Prostia Empire, it seems—the nation has experienced a rebellion of its own, and much of the imperial family has been killed or put in chains. What in the world happened? Can Allen and his party rescue the survivors? Or is all that awaits them a watery grave?