Orphan Henry's dream to see the ocean finally comes true when his Aunt Agatha plans a seaside vacation. But Henry's got more important things to do than sit on the beach and eat ice cream. He has a mystery to solve!

Henry can't sleep a wink his first night at Sibbald House because he hears moaning and groaning coming from somewhere inside the house. And when Henry discovers a hidden bedroom that makes the skin on the back of his neck prickle, he knows just who to call for help: Harvey Angell, the man with a five-hundred-kilowatt smile and Connecting Kit bulging full of curious gadgets.

The townspeople refuse to shed light on the mystery, but none of the lodgers are going to get any sleep until Henry and Harvey uncover the shocking secrets of Sibbald House.