They lie telling him they're going to meet an important senior. Iida notices the luggage is heavy. The next moment, he assumes he's carrying souvenirs they'll give to this acquaintance of theirs haha.

He starts questioning if it's polite to pass through places like this (sewers) They bullshit it's because you're showing your acquaintance the utmost respect by lowering yourself before the meeting them. (like, they are underground pfft)

The villains are in a hurry and knock down a civilian, Iida offers to help him but the thieves tell him to forget about it. They don't give a damn about a person falling, manners are more important (and they wanted to be punctual in their escape)

Iida yells that he believes discipline and manners were originally meant for the sake of others. He'll only follow etiquette if it gives priority to those in need! [that's why he helped them]

Their act is up, they gotta run. All Iida has been learning was a hoax! "Ingenium won't allow you to use manners for evil!"