Five Go To Mystery Moor by Enid Blyton

When Julian, Dick, Anne, George and Timmy spend their holiday on a windswept moor they stumble on strange lights, eerie noises and furtive strangers; determined to get to the bottom of it, the children camp out, follow clues across the boggy landscape, discover secret places and a criminal plot, and—by using courage, quick thinking and Timmy's help—unmask the villains and rescue those in danger.

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