Twelve-year-old twins Theodora and Alexander and their older sister Wilhelmina Sinister-Winterbottom don't know how they ended up with their Aunt Saffronia for an entire summer. She's not exactly well equipped to handle children. The twins are determined to make it a good vacation, though, so when Aunt Saffronia suggests a waterpark, they hastily agree. But Fathoms of Fun is not your typical waterpark. Instead of cabanas, guests rent mausoleums. The waterslides are gray tongues extending from horrible gargoyle faces. The few people they encounter are very, very odd. And the owner disappeared under bizarre circumstances, lost to the Cold, Unknowable Sea-the wave pool. When Wil goes missing, rule following, cautious Alexander and competitive, brave Theo will have to work together to solve the mystery of Fathoms of Fun. But are they out of their depth?