Clay Jensen is breezing through high school relatively drama-free, maybe because his nose is usually buried in his schoolbooks. Yes, he's upset by the recent death of Hannah Baker. But for the most part, except for Hannah's empty desk, his life is business as usual. Until, that is, he finds Hannah's package on his porch.
From the moment Clay pushes play on the tape recorder, we're plunged into Hannah's dark world as she sees it on the night before her death, when she records the tapes. She takes us on a psychological thrill ride that ends with her giving in to her despair. Her story is so compelling, so full of bitter twists, that most readers report devouring this book at breakneck speed. We are also driven forward by a need to see if Hannah is justified in blaming these people for her tragic decision. Clay is experiencing this same thing, but with a lot more at stake. He needs to find out why he's on Hannah's tapes.
All this can be seen as a coming-of-age for Clay. He has to move out of his comfort zone and grow as a person if he's going to survive these tapes. Since Clay is a responsible guy, Hannah's tapes make him feel like he needs to do the right thing in the world from now on. This means not listening to rumors or caring what others think about who he chooses to hang out with or date. But it also means confronting the not-so-pretty reality of his teenage world. No pressure, right? Hence the pounding headache he sports throughout his grim listening adventure.
Hannah reveals a dark and threatening world that was right there alongside Clay's peaceful, happy one. By the end of these tapes, the two worlds are shown to be one, and Clay will never be the same.
We know from the very beginning that this story is a tragedy. Heck, it ends with the death of our protagonist. But we should remember that Hannah's tragic death isn't in vain: in the end, it helps Clay (and hopefully other people on the list) become a more sensitive, caring individual. This tragic ending leads to a happy, new beginning.