On the run from an ancient evil and his army of terrors straight out of myths from around the world, Fi and Zeke aid Peter in his globe-trotting quest to seek out the remaining Firstborn, uncover the enemy’s plans, and gather the warriors of old for what may become the final battle in the world's oldest war. Along the way, Fi and Zeke discover they, too, have strengths of their own—though they come at a cost neither may wish to bear.