Spoiler alert: that's exactly what happens.
Hamlet, the play, is like the perfect story - it begins and ends at the perfect time. It begins: Hamlet's dad is dead, everything's in turmoil - it begins with a question: Who's there? It ends with one hell of curtain drop: everybody's dead. One of the big problems with Ophelia is that it starts way before the perfect beginning, and it ends way after the proper ending. It's unfortunate that Ophelia, like most of the women in Shakespeare's plays, has little character development. It's natural to want to tell more of her story, but I think where Klein goes wrong is trying to hard to match her novel to the play (there are some really lame bits, where, for example, Ophelia might wander by someone and overhear them saying something like, "The play's the thing!") whereby it's necessary for Hamlet and Ophelia to merely pretend that they are "mad." It felt as if she were fighting too hard against the play instead of just moving ahead with her own narrative.
In Tom Stoppard's Rosencrantz and Guildenstern Are Dead, he works with the play, not struggling to explain or work around the difficult sections, but embracing them for all their absurdity.
About half way through, it occurred to me that maybe Ophelia was meant to be a YA book, which, it turns out, it is. I don't necessarily think that a book that's meant for younger people is or should be inherently more simple or readable, and it irks me that this simple book is meant for teens. A dumbed-down version of Shakespeare benefits no one. I wouldn't recommend it to either young or older readers, simply because it's not intellectually satisfying. It doesn't elaborate on the themes of Hamlet, it doesn't offer new ideas, and the plot's beyond dumb.