When eleven-year-old Gwen ventures into the forest beyond her castle home she comes across the magical island of Avalon and her life changes forever. The lady of the lake, Nineve, asks Gwen to embark on a quest to protect the enchanted island of Avalon from the evil sorceress Morgana la Fay. Morgana has imprisoned the eight Spell Sisters of Avalon throughout the kingdom and stolen their magical powers. It's up to Gwen, her best friend Flora and a very special horse named Moonlight to find the sisters and return them to Avalon before its magic is lost forever. In this fourth adventure Gwen and Flora set out to rescue Amelia the Silver Sister who is imprisoned in an old castle. But when Morgana attacks the castle with a hurricane the girls become trapped too! Can they escape and return Amelia to Avalon?