As weird and wonderful as Escape from Mercy Hall and Secrets of Mercy Hall, the final book in the trilogy see the children back behind the Hedge again, this time on a rescue mission. Where is the lever to the Hedge? Shouted Jethro Barking as he towered over them! I'll feed you to the sharks, I'll hang you by your feet, I'll give you to the Muttons, one by one 'till you tell me. So ended Secrets of Mercy Hall. Now, Digby, one of the orphans, has gone missing after visiting a travelling circus. Whilst investigating behind the scenes Milly, Singer and Charlie discover a sinister secret. Minlings from behind the Hedge are caged and ready to be displayed in the circus freak show. The worst has happened — Jethro Barking, or Old Barking Mad, has finally escaped and is selling everything he can, gold, diamonds and Minlings! Aided by their old friends, Drago, and Batty the Tick-Tock bird, the children use powers gained in the Rainbow Cave to outwit Old Barking Mad and finally solve the mystery of the Hedge forever.

This weird and wonderful tale follows the exploits of eight year old Milly, and nine year old George and Samuel, as they escape from the grey world of a Victorian orphanage into a land of curious and unsettling modernity.