Die Walküre
Sieglinde enters and tells Siegmund a story about how, on the day she was forced to marry Hunding, an old man had come into the house and thrust a sword into the ash tree in the middle of the room.  They start to realize that they are brother and sister,  but it does not stop them from falling in love and they sing passionately about their love for one another.  Siegmund pulls the sword from the tree and the two lovers embrace.