Roger Two Hawks, an American Iriquois Indian, was shot down over Ploesti, Roumania near the end of World War II; only the Navigator, O’Brien, survived with him.
Then, strangely, they were in a foreign land, where no known 20th Century language was spoken, where the people seemed to be living in another time, where men carried bows and arrows.
Then the realization struck: somehow they had entered a parallel universe, through a temporarily opened gate in the atmosphere.