When a mysterious island emerges from the River Riddle, panic spreads through Owlchurch. It's the Island of Ghosts, where ghosts of past Rymers dwell in afterlife. Its appearance means only one thing - a living Rymer is in mortal danger. Bad news for Thomas Trew: he's the only known living Rymer. A plan is quickly hatched to send Thomas back to London where he is safe from the dangers of the Hidden World, but Thomas has other ideas. He loves living in Owlchurch and he can't bear to leave all his new friends. Thomas soon devises a daring plan to search the island for his mother, Mab, a past Rymer whose ghost must live there. A dark and dangerous adventure begins as Thomas enters the island's door.