There's a new pupil at St Agnes' school in the historic city of Meresbury. She's a mysterious, otherworldly girl called Emerald Greene. A girl without a past, without a family or friends. When Jess and Richie decide to befriend her, they're in for a few shocks – for Emerald lives in a mansion which doesn't exist, and can see things nobody else can see. Meanwhile, Meresbury becomes home to a series of strange events. What is the secret of the tomb which the eccentric Professor Ulverston has unearthed in an ancient stone circle? Where does the talking cat come from? And why have Mr Courtney and his Special Measures operatives arrived in town? With unearthly apparitions stalking the land, Jess and Richie soon realise that their mysterious classmate might be the only one to know what's happening. But it seems Emerald Greene has her own secrets – and nobody is quite sure whose side she is really on. . .