Adrienne and Dakota were friends since they were kids but Dakota suddenly drops her without explaining why. Adrienne is an average student with a small group of friends when her former best friend Dakota vanished. Everything is just weirder since Dakota called her a couple of days before disappearing. Some thought she killed herself some didn't but everyone eventually moved on while Adrienne couldn't let it go.
I wouldn't have any problems with A being obsessed Dakota treated her right. But she didn't. Dakota was an awful friend which Adrienne didn't see. Dakota was always this enigmatic girl who did what she wanted all the time and I think she A wanted to be just like her which we can see when she starts wearing similar clothes and starts acting like Dakota.
A starts ignoring her boyfriend and starts hanging around Julian, Dakota's boyfriend, trying to find out what happened. Why was she so determined to find the truth when Dakota was such an awful friend? Well, I think every one of us had a friend who wasn't actually a friend and we desperately tried to please them even though they were never good for us. They eventually find out what really happened and I'm glad they at least have each other in the end.