Oskar is convinced that his father, who died in the 9/11 attacks on the World Trade Center, has left a final message for him hidden somewhere in the city. Feeling disconnected from his grieving mother and driven by a relentlessly active mind that refuses to believe in things that can't be observed, Oskar begins searching New York City for the lock that fits a mysterious key he found in his father's closet. His journey through the five boroughs takes him beyond his own loss to a greater understanding of the observable world around him.