Complex sentence: The plot involves a San Francisco health inspector and his colleague who discover humans are being replaced by duplicate aliens who appear to be perfect copies of the persons replaced, but devoid of any human emotion, who attempt to install a tightly organised, conformist society.
Split into simpler sentences: The plot involves a San Francisco health inspector and his colleague who discover that human beings are being replaced by aliens. The duplicates, who appear to be perfect copies of the persons replaced, but are devoid of any human emotion, attempt to install a tightly organised, conformist society.