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 their third adventure Gwen and Flora must find Isabella the Butterfly Sister. But with wolves under Morgana's magical control stalking them through the forest, will the girls be able to find Isabella and return her safely to Avalon?