@Cernunnos
Overview (assuming you know the synopsis of the manga):
For no apparent reason, a large horde of monsters is coming to the town. Independently, two groups of heroes were tracking the horde;
• First, the two party members of the real heroes' party.
• Second, two other heroes, that for an unknown reason are pretending to be the other two party members. Hilariously, they're succeeding at this, despite being fakes.
They met up in the town, because hordes of demons stomping through helps no-one. Also, people are worried that the horde is unnatural, and both groups above are suspecting each other.
Why are they suspecting each other? Because they're both harbouring demons;
• The fake adventurer girl is a demon. She is hiding her presence pretty perfectly, and that's why her companion doesn't notice. Also, she has a hidden motive, something about a Grand Duke (ominous inner monologue page near the end). But the real heroes noticed. So she's under suspicion, but they're not worried about it all that much, because heroes.
• Our best girl and her brother are demons. They're also awesome, so they get a pass. The fake heroes noticed this, because they have former goody paladin with them (the dude). The demon girl also noticed, but she wasn't going to mention it. Because the paladin is a goody, he thinks demons are always evil, and that's why he's trapping them (last page).
Most confusing part about this chapter? The flashback to the heroes' meeting at the beginning. The actual story only starts after the first 'This journey is DOOMED.'