A twin sister -- who's in love with the man she loves....

When Catherine Casey falls in love with David Britton, she's sure her life has just become perfect. She's got a successful career, a wonderful adoptive family and now she's going to marry the man who is everything she wants.

When Shannon White falls in love with David Britton, he emphatically does not return the sentiment. She approached him at his Britton Institute, which does twin research, because she's recently learned she might be a twin, and hopes her unknown sister is looking for her. Her feelings for David become obsessive, and she convinces herself that he does love her.

David and Catherine are planning their wedding, and then -- Catherine meets Shannon. Her twin....