After escaping from Edgaren't and Dr. Jay, the Sinister-Winterbottoms and their friends arrive at Stein Manor Science Camp. They all work together to distract the friendly teenage camp director so they can explore and look for clues about where their parents might be.

Meanwhile, Theo and Alexander settle on a daring plan. They know exactly where Edgaren't is going to be: coming for them, and the books. This time, they'll be ready. This time, they'll get answers. And this time, they won't trust the adult in charge, which is easy enough when it's enormous, lurking, unfriendly Mr. Frank. But as they get closer to opening the books and their parents' histories, an unexpected foe is watching everything they do. . . .

Fake volcanoes! Real sea caves! Strangely modified frogs! Startling betrayal! And a lightning-struck manor! all combine for the electrifying penultimate Sinister Summer adventure.