Nine-year-old Oskar Schell is an inventor, amateur entomologist, Francophile, letter writer, pacifist, natural historian, percussionist, romantic, Great Explorer, jeweller, detective, vegan, and collector of butterflies.

When his father is killed in the September 11th attacks on the World Trade Centre, Oskar sets out to solve the mystery of a key he disovers in his father's closet. It is a search which leads him into the lives of strangers, through the five boroughs of New York, into history, to the bombings of Dresden and Hiroshima, and on an inward journey which brings him ever closer to some kind of peace.