Mahiro is the prince of Mismarca Kingdom, he won't study nor learn sword skills, he is a lazy prince who only plays until late night. His Imperial Bodyguard Pariel would always scold him for his behaviour. And towards that Mismarca Kingdom, an elite squad from the Demonic Kingdom Granmarsenal is approaching. The Leader of the squad is Lunas, a warrior princess, the third Princess of Granmarsenal. All of Mismarca's vassals voiced their will of fight and bring an all-out war, though there is one man said cowardly "Why don't we have a talk with them?". What kind of fate awaits Mismarca Kingdom and its useless Prince, Mahiro? Adaptation of the light novel series of the same name written by Tomaki Hayashi with illustrations by Tomozo.