Sabrina finds an unexpected roommate: Gwen, a British witch with a talking guinea pig named Stonehenge (nicknamed "Stony").

At the inn where she is staying, Sabrina learns that her Aunt Sophia was banished and trapped in the locket because she fell in love with a mortal who betrayed her and revealed to a friend that she was a witch.

When Paul and his friend Travis witness Sabrina doing magic, while turning a walking statue to stone (that Gwen brought to life by accident) they come up with an idea to sell the story.

The book adds in a subplot not shown in the film - featuring Sabrina's aunts back in Westbridge spying on Harvey and a mysterious girl named Tish.

This also references previous continuity from the TV series, where Hilda has brought back the three butlers the aunts used to employ (as shown in season 1's "The Great Mistake").