A mystery

A few years ago, when we bought the house , the garden seemed to have a limitless space to fill.  We went to nursery sales and neighborhood garden sales.  As you know, we've even acquired plants at garage sales.

 
But this is our mystery plant.  I think we've had this plant for almost ten years and I still don't know what it is.

I've asked two master gardeners  and still no idea.  We bought it as a small twig with a few leaves and a sign that said "anemone bush - grows to 5 feet".  That was so funny at the time because it stayed a tiny little couple of branches for at least three years.

Then it started to grow.  And grow - it's over 5 feet now and it's been pruned often.  Then one year, it produced a bud.  One.  It was a white flower which disappeared in less than a week.  The next year we had more.  Now, we have lots.  This is a gorgeous bush when it flowers.  The scent of the blossoms is stronger than our lilacs and is very pretty. 

Anyone know what it is?