To celebrate her birthday, George is taking Nancy and Bess on a three-day bike tour around the scenic Mystic Lake. They can't wait to spend time exploring the gorgeous landscape by day and camping under the stars at night. What they didn't count on was sabotage! First someone steals food and supplies from their packs. Then tour guide Caitlin vanishes overnight, and the entire group finds their tires slashed to bits. Now Nancy is cycling for clues instead of soaking up the scenery. Can Nancy track down the culprit before she and her friends are lost in the woods forever?