Imagine you're a German soldier, fighting your way deep into the Soviet Union during the summer of 1942. During the time you're not actually on the front lines, you feel pretty safe and even sleep a full night without constant vigilance.
Still, you can't relax completely. There have been whispered rumors of 'night witches' who fly silently after dark and drop bombs into previously safe areas, destroying military targets and fraying nerves. You don't completely believe it, but like a lot of rumors, you wonder if there's likely some grain of a truth.