A young boy named Oskar Schell is trying to cope with the tragic loss of his father, who died in the World Trade Center during the September 11 attacks. Oskar discovers a key in his father's belongings and becomes determined to find what it unlocks. This quest, sparked by the need to stay connected to his father, sets him on an intricate journey across New York City. He meets a variety of people along the way, each with their own stories and perspectives, which help him understand the world and cope with his grief.

Oskar's relationship with his mother is strained as they both wrestle with their loss in different ways. His quest leads him to uncover hidden layers of his father's life and, ultimately, deeper truths about himself and his family. Throughout his journey, he is accompanied by "The Renter," a mute man who helps him navigate the emotional and physical labyrinth he finds himself in. The story is a poignant exploration of grief, love, and the enduring human spirit's capacity for connection and healing.