The scarlet slipper mystery - Carolyn Keene

Nancy meets Helene and Henri Fontaine, refugees from Centorvia who run a dancing school in River Heights. Strange circumstances have brought the brother and sister to the United States. When they receive an anonymous note threatening their lives, Nancy offers her help, but she encounters nothing but puzzles. Are the Fontaines involved with the Centrovian underground? Have they been threatened by their own countrymen? When the Fontaines disappear, Nancy and her friends pursue the trail, even though danger lurks around every corner.