In the past, Ghosts were regarded dangerous and getting rid of them had a purpose. Nowadays, the poor things serve as shooting target dummies for a teenage girl, that got permission by her parents to try it out in the streets of New York. Of course, it has to be a nuclear gun, else it's not as fun.
The big evil guy, was so frightening in the past that an elite group of "ghost chasers" at the high of their skills had to contain it. They were not be able to take it down. Now, in the present, the ancestor of this group, who recently found out he somewhere has powers and can light up a candle with it, plus the aforementioned teenage girl are enough to solve the problem.
What's left for the viewer is to follow a strange romantic relationship between the teenage girl and a spirit, getting to know some other characters who don't really add anything to the story, endure some bad jokes and watch the cgi do what it is supposed to do.
By now, you have already given up, at making sense on all what is happening. Probably, that's the goal. You can't complain about plot holes, if all isn't serious in the first place, lol.
I would rate it as kid movie and it would get a higher rating, but since an adult watched it, 4 out of 10 is a good score. Additional remark, I want watch the next Ghostbusters, since I am not the target audience it seems.