Meet the Sinister-Winterbottoms: brave Theo, her timid twin, Alexander, and their older sister, Wil. They're stuck for the summer with their Aunt Saffronia, who doesn't know how often children need to eat and can't use a smartphone, and whose feet never quite seem to touch the floor when she glides--er--walks.

When Aunt Saffronia suggests a week pass to the Fathoms of Fun Waterpark, they hastily agree. But the park is even stranger than Aunt Saffronia. The waterslides look like gray gargoyle tongues. The employees wear creepy black dresses and deliver ominous messages. An impossible figure is at the top of the slide tower, people are disappearing, and suspicious goo is seeping into the wave pool.

Something mysterious is happening at Fathoms of Fun, and it's up to the twins to get to the bottom of it. The mystery, that is. NOT the wave pool. Definitely NOT the wave pool. But are Theo and Alexander out of their depth?