Die Walküre
The opera starts with a storm.  The young man Siegmund is out in the storm and comes to a house.  The house belongs to Hunding who lives there with his wife Sieglinde.  An ash tree is growing in the middle of the house.  Siegmund and Sieglinde are twins who were separated as children, so neither of them knows who the other one is.  Their father is Wotan, the chief god, but they don’t know this either.  Their mother was a mortal (an ordinary person, not a goddess) or perhaps a she-wolf, which is how the goddess Fricka refers to her.