Elly, Tash, and Sierra are back for their most mysterious adventure yet. At a sleepover in Tash's tree house, the girls are going through all of Old Man Blake's inventions, trying to figure out what they are and what they're meant to do. When Tash pulls out a ghost detector, the three of them can't resist trying it out. At first, nothing happens, but then it starts bleeping madly, and when Elly looks out of the window and sees lights coming from the graveyard, the girls know something's up. There are rumours that Sunday Island's haunted by the ghost of Old Man Blake, so perhaps he's trying to tell them something about his inventions . . . But Sierra's sure that ghosts don't exist. So why is she scared of them? And if ghosts don't exist, then who was it in the graveyard? Can the girls face their fears and solve the mystery?