Oskar (Thomas Horn), who lost his father (Tom Hanks) in the 9/11 attack on the World Trade Center, is convinced that his dad left a final message for him somewhere in the city. Upon finding a mysterious key in his father's closet, Oskar sets out in search of the lock it fits. Feeling disconnected from his grieving mother (Sandra Bullock) and driven by a tirelessly active mind, Oskar has a journey of discovery that takes him beyond his loss and leads to a greater understanding of the world.