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.

Dullahan, a top leader in the Devil King's army, yet again appears at the city in a frenzied rage.

The rampaging ancient weapon: the mobile fortress, Destroyer, is headed straight for Kazuma and party's city!

As a result of a magic circle that Aqua had drawn in Keele's dungeon, strange explosive doll monsters have started to appear, putting the town at risk.

Kazuma's incorrigible ego forces him and his unmotivated idiosyncratic party to accept quest after dangerous quest following his bragging in front of his first hardcore fan. This time, the adventurer's guild requests that they take out a giant golem guarding ancient ruins...