What hint does Williams drop in the opening scene about the story to come? What mystery?

To start, we have Blanche's surprise visit to her sister's home, a visit that looks like Blanche hasn't planned a departure date..... or simply doesn't plan to leave. We're also left confused as to Blanche herself; who is she; why is she there; what really happened to her job; how was the family plantation really lost? Blanche is our mystery. Stella and Stanley couldn't be more up front as to who they are, or what feelings they share.