The Knight of the Night (1953)

Paris, 1884. Bella Fontanges, a renowned ballerina, is married to Georges de Segar. But after just a few years together, the couple slowly sinks into weariness. Bella has forgotten what happiness used to taste like. Then a mysterious squire appears. Aware of the couple's faltering situation, he makes Bella and Georges a most astonishing proposal: he claims to be able to separate Good from Evil in Georges. Reluctant at first, the couple eventually agrees to play along. But it's not long before they realize that in Georges, it's Evil that has prevailed.