Oskar Schell (Horn) is a young man whose borderline autism with its attendant driving obsessions and discomfiting logic would try the patience of the proverbial saint.
So it's just as well that his dad Thomas (Hanks) is totally in tune with his son's demanding curiosity, able to amuse the youngster while hatching ever more extraordinary puzzles for the enthusiastic little chap to work out.
When Thomas dies in the September 11 attacks, to his shell-shocked son there is no logic - and his intention to apply a solid rationale to a senseless act drives him to extremes of behaviour.
Months after the death, Oskar discovers a key hidden in an envelope marked 'Black' in a vase in his father's wardrobe. Springing into cathartic action, Oskar compiles a list of everybody in New York called Black and puts together a rigorous classification system in a bid to visit them all to ask about his dad.
Initially attempting the awe-inspiring task on his own, he ends up recruiting the mysterious mute German refugee (Von Sydow) who rents a room at his grandmother's apartment building across the road from his own.
Adapted by Eric Roth from the novel by Jonathan Safran Foer, director Stephen Daldry has put together a richly inventive drama fired by Oskar's restless energy and given an extra dimension by those people - the Blacks - he runs into during hsi quest.
Sandra Bullock impresses as Thomas's quietly grieving mother and Hanks delivers a quirkily appealing performance as the doting dad, but it is Horn who steals the show, his almost physical frustration at the manner of his father's passing the well-spring of the narrative.
Although running the risk of slipping into mawkish melodrama by the end, this still has the power to move.