11-year old Oz Chambers lives in a haunted house. His mother wants to move, but Oz would rather do double algebra (yuck) every day for twelve months than leave. Where others see spooky, Oz sees wonder and mystery and aching reminders of his deceased dad. When he and his friends hear ghostly footsteps in the boarded-up dorm at Halloween, it leads to an exploration of the old place's eerie reputation. In his Dad's locked study, Oz finds a parcel addressed to him and posted by his dad the day before he died. Inside is the obsidian pebble, a technology of astonishing scientific power and a clue to Penwurt's secrets. Suddenly Oz begins to change; he goes from maths dunce to A student overnight and has to deal with suspicious teachers and jealous pupils. But the footsteps in the locked rooms don't go away and slowly, Oz begins to knit together the strands of lies and mystery that tie the obsidian pebble, his father and him together. What Oz hasn't bargained for is that he's not alone in that search for understanding and that solving Penwurt's puzzles lead to other, much darker secrets that will test his loyalty and his bravery to the limit.