She is suffering from Dissociative Identity Disorder (DID), a condition characterized by having vivid alternate personalities that manifest in a person and are usually formed as a coping mechanism for an early childhood trauma. The actual particulars of Dylan's trauma take a while to be uncovered and the real itself is anticlimactic. The story focuses instead on Dylan's growing understanding of her condition and her learning to cope with it. Unfortunately, this part of the book (roughly the middle) is also the weakest section.
I was really captivated by the story from the start and reminded of how much I enjoyed novels dealing with mental health. There's a compelling mystery with all sorts of interesting elements that are slowly revealed. Up to the diagnosis, this is a real page turner. But once we know what is happening, the pacing really slows down and becomes this big educational text where we're introduced to Dylan's "system" and her "alters" who "front" for her from time to time because of various conditions. Not much actually happens in these 150 or so pages beyond a bunch of repetitive and strikingly boring conversations. It's only when the culprit (a completely new character never mentioned prior to that point of the story) is revealed that the pace picks up again. But here McLaughlin is at a loss as to how to portray the moment of confrontation and the last sixty pages reads more like a lengthy postscript than a climax. There is no dramatic conclusion. In fact, there really is no conclusion at all.
A fascinating topic but the presentation sucks the life out of the story. It starts strong but then treads water, before dying at the end with a whimper.