The Secret at Lone Tree Cottage — Carolyn Keene

Miss Tisdale, Jean and Louise Dana's favorite teacher, disappears mysteriously one afternoon. The Danas search for clues and discover her car abandoned on a nearby road. The key is still in the ignition, and there are signs of a struggle. Miss Tisdale has been abducted!

Jean and Louise learn that Miss Tisdale has been supporting her widowed twin sister, who has been disowned by their parents ever since she married. Mrs. Brixton is being threatened by her husband's former partner, and the Danas suspect that this man is responsible for Miss Tisdale's strange disappearance.

The police cannot be contacted, because Miss Tisdale's father is in frail health and the shock of her disappearance could kill him. The Danas seek Uncle Ned Dana's help in tracking down Miss Tisdale, and the three Danas undertake their difficult mission without assistance.