So I watched the movie in anticipation of the review, and one thing I feel like adding is... McCarthy's character is an asshole. And not the lovable kind of asshole like Venkman, she's a complete piece of shit.
So the backstory is... Wiig's character saw a ghost when she was a kid, and nobody believed her except McCarthy, who became her only friend. They wrote a book together (which is nice), and eventually stuff like therapy forced Wiig to try and put ghosts behind her, and that apparently caused a breakup.
Setting the backstory aside, the movie begins with McCarthy violating Wiig's half of the book they wrote together by publishing it, using Wiig's name and picture, against Wiig's explicit non-consent (Wiig says she "burned both copies", aka one copy was made for Wiig and another for McCarthy. Wiig revoked and then burned McCarthy's copy, but not before McCarthy secretly made a third copy), simply because McCarthy wants a new mini-fridge for her lab.
This apparent cry for attention forces Wiig to come around to try and speak to McCarthy. Nope, McCarthy tries to throw her out of the building.
Wiig explains that this intrusion is badly timed and that it could ruin her shot at tenure, aka the green light which allows her to research anything she wants at the university's expense, without getting fired. Rather than seeing what a gift that could be for their friendship and their fringe research, if only McCarthy temporarily stops violating Wiig's rights for a fucking fridge, McCarthy says "Nope, fuck your life. Now step over here. Hah! Queef joke! For a moment there, I really had you interested in paranormal science again. Fuck you."
And then there's the lead Wiig drops in McCarthy's lap. Nope. Fuck you, you can't come along. McCarthy is not interested in restoring their friendship or Wiig's interest in the paranormal. Wiig only tags along by blackmailing McCarthy, pointing out that they need Wiig to gain entry.
Then a ghost pukes on Wiig, and Wiig conclusively believes in the paranormal again. Yay! Conflict resolved? McCarthy posts Wiig's face all over youtube without telling her, and not only costs her the shot at tenure, but now Wiig is straight-up fired.
McCarthy: "That's fine (that I ruined your life without asking, when you asked me to do the exact opposite). Just join us in this shithole university. They throw money at anything you want" (except fridges, apparently). Oh wait, McCarthy is wrong, and now she's thrown out too.
This is apparently the end of their mended-relationship story, until the very end of the movie when Wiig literally jumps into hell to pull McCarthy back out, because McCarthy is such a precious friend. Is that the ultimate resolution that this movie needed to run with? That McCarthy is such an amazing friend that Wiig needs to go to the ends of the Earth to beg McCarthy's forgiveness for that fight that we never saw but can infer was about Wiig giving in to the weight of the world and being wrong about ghosts not existing? Also, why does McCarthy believe so strongly in ghosts anyways? Was it because her willingness to believe helped her form a bond of friendship (that was too weak to survive without their exclusive shared interest)?
Mr Plinkett pointed out how all the background characters in the movie were shit, making it hard to care if the world lives or dies, but the movie literally asked McCarthy's character what was worth saving in the world, what was worth living for, and all McCarthy could come up with was that weaksauce soup from the Chinese food restaurant, which the movie went out of it's way to hammer home was pathetic. That's how terrible McCarthy's character is. She just reunited with her best friend, and the only trace of positivity she can see in the entire world is a lukewarm cup of yellow liquid.