In Honolulu, a DC-4 airliner operated as TOPAC Flight 420 prepares to takeoff for San Francisco with 16 passengers and a crew of five. Former captain Dan Roman, the flight's veteran first officer known for his habit of whistling, is haunted by a takeoff crash in South America that killed his wife and son and left him with a permanent limp. 420's captain, Sullivan, suffers from a secret fear of responsibility after logging thousands of hours looking after the lives of passengers and crew. Young second officer Hobie Wheeler and veteran navigator Lenny Wilby are contrasts in age and experience. Stewardess Spalding attends her passengers, each with varying personal problems, and befriends the terminally ill Frank Briscoe after being charmed by his pocket watch. A last minute arrival, businessman Humphrey Agnew, soon causes her misgivings by his strange behavior.