Lilli Marlene.

Lilli Marlene, a French girl working as a bar maid in her uncle's café in Benghazi, Libya, turns out to be the girl that the popular German wartime song Lili Marleen had been written for before the war, so both the British and the Germans try to use her for propaganda purposes - especially as it turns out that she can sing as well. When the Germans kidnap her in Cairo and she starts appearing in radio broadcasts from Berlin, her British soldier friends think that she's joined the enemy. They couldn't be more wrong, because after the war it turns out that her songs over the radio contained secret messages to London from British agents in Berlin.