The film assembles an impressive group of performers—Flora Robson, Phyllis Calvert, Jean Kent, Patricia Roc, and Renee Houston—who portray women in a WWII internment camp with convincing dramatic depth, working to aid British airmen while avoiding detection by their German captors. What distinguishes this from many other films depicting wartime fortitude is its surprisingly upbeat tone. It isn’t overtly humorous, but…