Nancy meets Helene and Henri Fontaine, refugees from Centrovia who own 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 to help.

But she encounters nothing but puzzles. Are the Fontaines involved with the Centrovian underground? Have they been threatened by their own countrymen? Why? Is a series of paintings by Henri Fontaine being used for a sinister purpose?

Suddenly the Fontaines disappear. Have they been kidnapped? Nancy and her friends pursue the trail relentlessly, even though danger lurks around every corner. They are trapped by their enemies, and escape seems impossible. But Nancy's quick wit finally enables her to solve this intriguing and intricate mystery.