"Blue bells will be singing horses!" This strange message, attached to the leg of a wounded homing pigeon, involves Nancy in a dangerous mission. Somewhere, an elderly woman is being held prisoner in a mansion. Nancy is determined to find and free Mrs. Eldridge. Nancy also stumbles upon a related mystery at nearby Sylvan lake with the help of pal Helen Corning. In the end, Nancy helps foil a ruthless ring of swindlers but meets danger along the way.