During the Japanese invasion of China, a truck transporting civilian passengers by night, on the Assam Road to India, is forced to stop because a Chinese munitions truck is stuck in the muddy road. All lights are turned out when enemy Japanese bombers fly overhead, but Albert Pasavy, a trader, accidentally flips the headlights on while trying to ignite the cigarette lighter. The Japanese see the lights and bomb the road. The munitions truck is destroyed, so Lt. Tang commandeers the other truck to transport his now-wounded men and takes everyone back to a secret air field in Chungking. American captain Nick Stanton, who is attached to the Chinese Air Force, is furious that civilians have been brought to the secret base until he meets Ann Richards, a beautiful American Red Cross nurse. Woman-starved Nick cannot resist kissing Ann, but later resents her insistence that she and her companion, Madame Wu, who is on a secret diplomatic mission, be flown immediately to India. Nick flippantly recommends that Ann wire the minister of the interior for permission, but after he catches passenger Countess Olga Karagin in his office embroidering a map showing the secret bases along the Assam Road, he insists that they all return by truck to Chungking. Olga is arrested for spying, and as the passengers unwillingly board the truck, Ann gets a wire from the minister of the interior ordering Nick to fly the passengers to India. Nick grudgingly agrees, but en route, the plane is shot down by Japanese bombers. Nick manages to land the plane safely in the jungle and only his co-pilot, Capt. Po, is injured. Before the radio fails, Nick and Po learn that Olga committed suicide but revealed before her death that her superior is still among them. Nick and Po plan to build a raft to float downriver, but the next morning, the Rev. Doctor Van Der Lieden is found missing. Just as the group is about to embark on the raft, Van Der Lieden returns with a basket of food he claims he received at a nearby monastery. Fearing exposure, the passengers disobey Nick's command that they continue by raft, and follow Van Der Lieden on the three-hour hike to the monastery. Once there, however, Van Der Lieden reveals that he is a Nazi commander working with the Japanese forces. Nick negotiates for the safety of the hostages, and Van Der Lieden agrees to allow Po to repair the plane if Nick wires his superiors to trade Olga for the hostages. To stall for time, Nick sends the wire, knowing that Olga is dead. The return wire is encoded, and Nick pretends that the hostage exchange is proceeding according to plan. In the meantime, the male hostages are kept in cells, while Ann and Madame Wu are held in a chapel. When Van Der Lieden finally decodes Nick's message and reads its true contents, Nick and Maj. Raoul Brissac, who is with the French Free Legion, try to escape, but Pasavy, concerned for his own life, betrays them to Van Der Lieden. Van Der Lieden watches Pasavy grovel before him and then coldly shoots him. Nick kills Van Der Lieden, and after he helps free the women, they all escape under enemy fire. Japanese troops follow them into the jungle, where Brissac sacrifices his own life to save the others by pulling the pin on a grenade, killing himself along with the Japanese. Nick, Po, Ann and Madame Wu then fly to safety. Having fallen in love, Nick and Ann vow to reunite after the war.