When the class is informed that Kazuma, Aqua, and three of Tanya’s underlings have been given “detention”, it’s natural for them to be curious about what form their punishment will take. When Darkness encounters them in a hallway, they look like they’ve been scarred for life, which only makes Darkness more curious (not to mention jealous!)
As we learn, detention duties have been left to Nazarick Area Guardian Kyouhukou, as the offenders are thrown into the roach pit, where instead of eating them alive, the bugs simply crawl in and out of every one of their orifices. Naturally, their friends consider the punishment too severe for the crime of stealing booze, and wish to reduce their sentence if possible.
Emilia, the nicest and most “good” of all of the students, makes a passionate appeal to Mr. Rerugen, who decides that the punishment will be reduced if the offenders do a good deed. Their classmates then create a situation during a lab experiment that, while providing an opportunity for that good deed to be done, is still innocent enough on its surface to be overlooked by Roswaal.
In practice, this means allowing Megumin to light a Bunsen burner. Even though steps are taken to control her Explosion, the entire lab table within the bounded field is obliterated, and the broken pipes soon flood the school. Roswaal takes the hint that there was no malice in Megumin’s act (it’s the only magic she has!) and that people just have “bad daaaaays.”
When the water threatens the principal’s cigar stash, he agrees to lift detention if the offenders can solve the flooding problem. Grantz, Neumann and Koenig capture the water with their boundary magic, which allows water goddess Aqua to finally show that she is, in fact, a water goddess, redirecting the water out the window Kazuma opens (he did his part!) and dispersing it into the sky as a lovely rainbow.
Not only was this a great showcase for various characters powers, but a nice capper to a serial “two-parter” in which the classmates have each other’s backs when school discipline goes too far. Finally, it’s a reminder that no matter what isekai show you’re from, nobody (other than Darkness of course) wants bugs crawling all over their bodies!