When Mary-Kate, Ashley, and their basset hound Clue go to Camp Barkaway, a special camp for dogs and their people, Clue disappears from their cabin and the Olsen twins must find her.
While working as a baby sitter for two children living with their grandmother in an old "castle," fourteen-year-old Gail becomes suspicious of the accidents that befall the children and the strange noises heard at night from the locked tower.
When something destroys Lindsey's picture-perfect tulips for the upcoming flower show, all clues point toward a real, live unicorn, but Mary-Kate and Ashley have a little trouble believing it.