Eleven-year-old Zelly Fried's parents will not allow her to have a slumber party until she teaches her mischievous puppy, Ace, to behave, but with Grandpa Ace around nothing is ever simple.

Twelve-year-old Addie should stay away from Maple Lake. After all, her twin brother, Amos, drowned there only a few months ago. But its crisp, clear water runs in Addie's veins, and the notebook Amos left behind -- filled with clues about a mysterious creature that lives in the lake's inky-blue depths -- keeps calling her back. So despite her parents' fears, Addie accepts a Young Scientist position studying the lake for the summer, promising she'll...