Descent into madness. No, it's really not a fun concept to contemplate. Start with a lady finding a new (as in "another", not "fresh") apartment in a bland, mundane, decaying part of NYC. Add a missing girl, ambivalent clues about her existence, a curious child, vandals, and a plumbing leak that won't quit, and let all that slowly push her over the edge. ...or is something supernatural going on? Whatever the truth of the ending, it's not a recipe for a good time.
Doesn't help that the key tragedy in the plot was all too similar to a real-world crime that ended somewhere I worked. Look up Kali Ann Poulton if your morbid curiosity is too much, and you don't mind movie spoilers.