Jem Halliday has been in love with her best friend, Kai, for years. The only thing is, he's gay, which doesn't help matters but she's got used to this. However after he is cruelly outed online, and not just outed but exposed in an intimate position, he kills himself. Jem is devastated and wants to join him, however then she receives the letters. One for every month of the first year without Kai. Her plan changes and she decides to delay her demise so she can exact revenge on those responsible.What follows is an intriguing and disturbing novel. Jem is utterly obsessed both with Kai and revenge and her behaviour throughout the novel was very self-destructive to me, however Clarke brings her vulnerabilities and humanity out as well and incredibly effectively. Clarke creates human, multi-dimensional characters well and her narrators are flawed, imperfect and in Jem's case unlikeable, but at the same time vulnerable and relatable.Early on in the novel, Jem says something that for is unforgivable. I won't spoilt it, but it's a lie I don't think that can ever be justified, even in revenge. In all honesty at this point I wondered if I could finish the book as I was so furious at Jem, but I love Cat Clarke's writing so persevered and was glad I did. This lie just proves how much of a spiral Jem is in and how obsessed she has become with revenge.Parts of this novel like Jem's attempts to blend into the popular group's circle were really interesting to read and reminded me a little of Mean Girls. Clarke effectively exposed the superficial coatings of appearance and we group people by clothing, makeup, etc. The revenge plots start off fairly tame but become more and more sinister as the novel progresses.Clarke was directly influenced for Undone after reading an article about a teenager who was outed and killed himself. Undone is set before It Gets Better and the moment where Kai wishes he wasn't gay on the eve of his suicide is one of the most painful scenes and heartbreaking moments I have read. Kai's letters in general were incredibly poignant and the whirlwind of emotions he goes through as he writes all twelve on the eve of his suicide is a horrifically effective device and also constantly reminds the reader of just what Jem has lost.The ending in particular of this book is one that left me torn between throwing the book against the wall (don't worry, I'd never hurt a book) and crying. Days later, I was still haunted by Undone which is just a testament to how brilliantly written and provocative Undone actually is.Unflinching and uncomfortable, Undone may be Cat Clarke's best novel, and certainly one of the most important in terms of topic, yet.This review will go live at www.chooseya.com/2013/01/05/review-undone-by-cat-clarke/ in full on January 5th 9AM, UK time.