All of the weak monsters have gone into hiding since a leader of the devil king's army has settled in near town. Kazuma, who had been focusing on improving his skills, is stuck at a standstill. With nothing else to do, Kazuma tags along with Megumin's training, which consists of unleashing explosion magic into an abandoned castle.

Annoyed that Megumin had still been using his castle for explosion practice, Verdia, after proving easily susceptible to Aqua's purification magic, summons an undead army which is quickly defeated by Megumin's explosion magic.