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In Chungking, China, during China's war with Japan, schoolteacher Lin Ying secretly leads a double life as a spy for the Chinese army. While teaching class one day, Lin is summoned to meet another agent, who has an assignment for her. Lin's mission is to ensure the safe passage of a secret food supply wagon caravan across the Burma Road. When Lin returns to the classroom, the city comes under aerial attack, and although she tries to take all the children to a bomb shelter for safety, one boy is left behind and dies in the attack. Beginning her assigned task, Lin boards a bus traveling down the Burma Road. Among the other passengers on the bus are Me-Hoi, a Chinese serviceman posing as a priest, and Sir Roger Howe, a German spy. When the bus driver, Slim Jenkins, comes upon a destroyed bridge, he has the passengers disembark and wait at a nearby hotel. Soon after it is discovered that the bus's distributor is missing, war planes are heard in the distance. When Roger discovers that Me-Hoi and Lin are in the employ of the Chinese government, he arrests them and puts Slim in charge of keeping watch over them. Slim holds Lin and Me-Hoi captive for a only short time before they are able to convince him that Roger is the one who should be arrested, as he has been sending secret messages to the enemy about the convoy's whereabouts. To prove that her suspicions about Roger are true, Lin challenges Roger to ride with her in the first truck of the supply convoy, knowing that if it were to come under attack, it would surely be destroyed. Roger consents to ride with her, but the moment he hears the sound of the approaching bombers, he jumps out. Lin prevents his escape by signaling nearby field workers to surround him. The Chinese workers trap Roger and kill him, after which the Burma Road is safely reopened to the food supply caravan.