Recently widowed Berlin-based American aviation engineer Kyle Pratt takes her husband David's body back to the U.S. with her 6-year-old daughter Julia, aboard an aircraft she helped design, a brand new Elgin E-474 (loosely based on the Airbus A380 with the aircraft's classification number being similar to the Boeing 747) operated by Aalto Airlines. Awakening from a nap, Kyle finds Julia is gone, and none of the passengers or crew recall seeing her. Flight attendant Stephanie tells Kyle there is no record of her daughter boarding the flight, and Kyle is unable to find Julia's boarding pass and backpack. At a panicked Kyle's insistence, Captain Marcus Rich conducts a search of the aircraft, while sky marshal Gene Carson monitors her.
Kyle reveals that her husband died falling from the roof of an avionic manufacturing building, which she doubts was suicide. Captain Rich receives a message from a Berlin hospital that Julia died with her father, and is convinced that Kyle, unhinged by her husband's and daughter's deaths, imagined bringing Julia on board. The increasingly erratic Kyle is confined to her seat, where a therapist, Lisa, consoles her. Kyle doubts her own sanity until she notices the heart Julia drew in the condensation on the window next to her seat.