This romantic story of a Polish concert pianist/fighter pilot was never really very good. The dialog is often less than sparkling, the sets tend to look cheap and the few outdoor shots are often just painted backgrounds. There is an impressive shot of Spitfires taking off to do battle but the climatic dog fight is rather poorly choreographed. The script is rather lumbering with a couple of non too subtle nudges at American, Irish neutrality.
Anton Wallbrook as the pianist is his usual suave self, only sometimes he is not too convincing when he is supposed to be playing the piano, also I am sure that no refined European gentleman would have a conversation with a lady (especially one as beautiful as Sally Gray) with a cigarette dangling from his lips. That was probably the director's fault.
What makes this film memorable of course is the music. One of the most famous movie scores in history. Even now "The Warsaw Concerto" is a standard item on pop-classical concerts.
Just a few words about Sally Gray. She was one of the most beautiful and seductive actresses in British movies during the forties who was generally wasted in her roles. Perhaps it was her personal choice but I was surprised that she was never snapped up by Hollywood. I always thought what a great Hitchcock heroine she would have made. Alas, it never happened.