It's alien invasion humans trying to fight off in s1 and s2, in s3 suddenly there is a cult who thinks alien is their mother (LOL) and will make them live forever or see those who are dead in memory form (LOL2). Show could have used proper invasion story but lost it to a boring stuffs.
Thing I wonder, why Alien stories always end up being Hive mind? (Let's kill/destroy that thing and everything will be over) Hmmm.