After a routine departure, 420 experiences sporadic sudden vibrations. Although the crew senses that something may be wrong with the propellers, they cannot locate a problem. When a vibration causes Spalding to burn her hand, Dan inspects the tail compartment but still finds nothing amiss. After nightfall, as the plane passes the point of no return, Agnew confronts fellow passenger Ken Childs, accusing him of having an affair with Agnew's wife. The men struggle and Agnew pulls out a gun, intending to shoot Childs, but before he can do so, the plane swerves violently when it loses a propeller and the engine catches fire. The crew quickly extinguishes the fire, but the engine has twisted off its mounting. In mid-ocean, the crew radios for help, assisted by an amateur radio operator aboard the steamer "S.S. Cristobal Trader", and sets in motion a rescue operation. Dan discovers that the airliner is losing fuel from damage to a wing tank and that as a result, along with adverse winds and the drag of the damaged engine, the plane will eventually run out of fuel and be forced to ditch.