The secret of Elizabeth

Kate and Allan Royce are driving home from a party in Westport, Connecticut one night when they see a girl in a beautiful but muddied dress wandering in the road and stop to pick her up. She is suffering from amnesia, so they name her Elizabeth X and take her into their home while the police try to establish her identity. They are left in peace, until first a couple claim her as their daughter and then a psychiatrist arrives to say that she has escaped from his clinic. What does seem certain is that she is the child of wealthy parents. But who really is Elizabeth X, and what has happened to her?