What is the plot of the book?

Extremely Loud and Incredibly Close, Jonathan Safran Foer's ambitious second novel (after Everything is Illuminated), follows nine-year-old Oskar Schell as he navigates New York City on a quest to unlock the secrets of a mysterious key and its connection to his father, who died in the collapse of the World Trade Center on 09/11. Most of all, the novel is an exploration of grief set against the cultural backdrop of post-09/11 America, interspersed with secondary narrations by Oskar’s grandparents, whose lives parallel their grandson’s in significant ways. Typically, one of the secondary narrators provides one chapter for each chapter of Oskar's narration. Grandma's story is told via letters she writes to Oskar, whereas Grandpa's letters have been written over the course of decades, and are all addressed to his son (Oskar's father, Thomas Schell Jr.).

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http://www.gradesaver.com/extremely-loud-and-incredibly-close/study-guide/summary