Hiiro is the only survivor of an ogre mercenary band ambushed by Clayman's forces, saved and named by Towa, the young queen of the small country of Raja, whom he has served ever since. Raja's lake runs poisoned, and the inherited tiara Towa uses to purify it is draining her life, so Hiiro sets out to find another way. His search crosses Rimuru Tempest, the slime turned Demon Lord of Tempest, and the two follow the trail into a conspiracy closing around Raja's throne. This theatrical film is set after the second TV season.