Complex sentence: Moses sees Paul while the alien is trying to collect Clive's passport (that he left with Ruth after parking), and, believing him to be a demon, grabs his shotgun and chases after them in his truck.
Split into simpler sentences: Moses sees Paul while the alien is trying to collect Clive's passport (that he left with Ruth after parking). Believing him to be a demon, grabs his shotgun and chases after them in his truck.