We pick up right where the last volume left off. Megumin begins rapid firing Explosions at the castle. The demon army falls into a panic and begins to send out troops to try and stop the explosion onslaught, but all get Exploded before they even get close. Eventually, some sort of angel claiming to be the strongest magic caster appears, but gets exploded. He survives but his clothes don't. Eventually, he retreats back into the barrier and begins to strengthen it, aiming for a war of attrition. Megumin fires 10 Explosions in a minute, breaking the barrier and wiping the angel off the face of the earth. (We never learn his name).
Kazuma and the gang quickly meet up with Yunyun and Mitsurugi, who began running towards the entrance after they hear the explosions. (They think that the demon lord saw Kazuma and co. and began attacking, until Kazuma spills the beans that it was Megumin. Yunyun is not pleased, seeing as she and Mitsurugi's party could have become collateral damage.) Aqua managed to get lost while everyone was running.
While looking for Aqua, Kazuma finds a big red button that says "Please don't push". He detects a trap, but figures that the button is actually a secret passage that leads to the Demon King's room. The trap is a minor one, just to hide the purpose of the button. He pushes it, and finds Aqua in the place he teleports to.
Aqua is super happy to see Kazuma. The two of them debate just teleporting back to Axel, but Kazuma doesn't think the rest of the party will turn back without finding them. They hear voices coming closer. Kazuma manages to find a pot to hide in, but it's full of water. Aqua tries to take the pot as a hiding place (she can breathe underwater), but Kazuma tips over the pot to spite her. The two are immediately discovered. (Kazuma tries to use presence concealment but there's a wraith that sees Aqua's godly aura, so that goes out the window.)
It looks pretty bad for the two idiots, but Aqua prompts Kazuma to blind his foes, which he does with a new "Flash" skill he got ... only he blinds Aqua too, because she was expecting a dust trick like he usually does. The wraith (named Pain) doesn't have eyes either, so he's not blinded either. The Wraith and Kazuma fight, bonding over "Drain Touch" tastes, but eventually Aqua god blows the wraith after she gets her vision back. Kazuma ends up teleporting the other goon back to Axel, leaving only his armor behind. (Which was immune to teleportation for some reason).
Kazuma takes the goon's armor and pretends to be him (aided by the buff to make him a party trick god). He pretends to capture Aqua, and reports to the boss of the floor, suggesting they use Aqua as bait to lure out the other intruders. (Everyone in the demon army is suddenly grossed out by what low-brow villany their comrade is stooping to all of a sudden). While the army wait for the intruders to be caught, they interview Aqua about the intruders. They even give Aqua tea, which she of course purifies several times.
Aqua hypes the hell out of Kazuma causing all the demons to freak the hell out, just in time for the party to arrive. In order to distract the demons, Kazuma (in disguise) begins to bully Darkness. As the other demons recoil in disgust, Kazuma begins to force Darkness to strip off her armor. This distraction works well enough that Kazuma is able sneak up behind the bosses back and backstab him with a poisoned dagger. The party manages to slay all the demons there in the ensuing chaos.