In Los Angeles, air hostess Rita Moore, Jean Muir, chats with her suitor Terry Brewer, Warren Hull, as passengers check in for a flight to New York aboard a Ford Trimotor that is about to become a crowded twelve-seat ordeal. Brewer quickly realizes that federal agent Mike Phelan, John Litel, is traveling incognito among the travelers, and he suspects Phelan will be chasing the notorious killer Madsen, Howard Phillips. To prep for what he expects, Brewer phones his editor and boards the plane along with Rita’s other suitor, pilot Bob White, Gordon Oliver, who has his own eye on Rita.
Shortly after a stop in Albuquerque, the quiet passenger Katie Tristo, Nedda Harrigan, strikes up conversations about astrology and buys a dagger from a local stand. Between this stop and the next in Kansas City, the mood on board shifts when a passenger, Ramon Duval, Bill Elliott, is murdered with the dagger as the plane crosses the southwestern landscape.
When the death is uncovered, Phelan reveals his true identity and tries to take charge, but a bold “female” passenger draws a gun, disarms him, and handcuffs the undercover agent, hijacking the aircraft. The disguise comes off: the hijacker is indeed Madsen. He disables the radio transmitter, discards the pilots’ guns, and orders the crew to fly toward Evansville, Indiana. Brewer conducts a brief interview with Madsen, who insists he never killed anyone with a knife, leaving the passengers to wonder who is the real killer among them.
Rita attempts to drug Madsen’s coffee, but instead knocks out copilot Johnny Martin, Carlyle Moore Jr.. A dust storm erupts, forcing an emergency landing in Missouri, where Madsen orders White to keep the plane aloft until the storm subsides. He then departs with the elderly couple whose farmhouse he commandeers, taking several passengers hostage at a nearby home while the rest of the group finds another residence to ride out the danger.
As the escape continues, White is forced to repair the engines and resume the flight, with Brewer volunteering to help. While Madsen keeps a wary eye on White, Brewer manages to repair the radio and file a news report, all while the hostages huddle in a house that has no telephone.
Back in the cabin, the group examines anaglyph 3-D photos with a stereoscope, and Rita notices that Katie appears younger when viewed through its red lens. Phelan deduces that the second fugitive is Katie Dorne, a disguise worn by Katie Tristo. Rita finds a way to arm herself and confront Madsen, but he uses Brewer as a shield and shoots White, wounding him. Brewer then manages to overpower Madsen.
Phelan arrives, retrieves his handcuff key, and cuffs Madsen, while also arresting Dorne for murdering Duval. The two criminals had stolen bonds worth about $90,000, and Phelan had been tailing Duval to trap them both; Madsen’s arrival on the same flight was a coincidental twist of fate. With the danger behind them, White offers a quiet smile as Brewer declares his love for Rita; the couple shares a kiss as the plane, once again under control, heads toward its destination.
