Arthur is a private investigator.
He used to be in the military, but because he is a conspiracy theorist of the worst kind, they respectfully kicked him out.
Of course, he's convinced that this is a conspiracy to get him out of the way, so he became a private investigator in search of the Truth.
Now, he is on the trail of aliens. Men from Mars, no doubt.
They are the root of all evil, as far as he's concerned.
They may look like humans, they may act like humans, and maybe nobody believes him, but Arthur knows they're aliens.