In this third installment of the Lizzie series, we find an older Lizzie Lipinski ready to work away from her family for the summer. After working for the Wrightway family's hotel for a few weeks, and after making their son Peter's acquaintance, Lizzie becomes curious about an anonymous and seemingly captive figure locked in the turret room of the hotel. With help from the hermit, residing in the nearby woods, Lizzie plans to discover the secret of the turret room. She enlists Peter's aid and together they embark on the big lake mystery. Can Lizzie and Peter succeed in their plan to expose the mystery and change the Wrightway family forever?