Claire and Luna are thrilled to visit an ancient castle in Scotland with their five-star witch grandmother, Grandy. But this is no ordinary vacation. The three witches are on a mission: to rid Glenn Bly castle of the terrifying ghost that haunts it. The situation at Glenn Bly is more complicated than the twins had imagined. There's prickly Daphne, who lives with her grandfather, Mac, in the castle and seems to be hiding something. There are the awful Shrillingbirds, the absentee landlords who now want to make Glenn Bly their permanent home. Finally, there is the ghost itself. And when Claire and Luna meet him, the real mystery begins. . . .