Well, it's about a girl called Jess who has just moved to an island called Lume. She finds an abandoned cottage one day, and she feels someone is near her, even though it is empty. As the days go on she goes to the cottage often, where she finds a great many clues, and she also finds diary entries made by a boy who lived 100 years ago called Sebastion Wren. As the plot thickens mysterious stuff happens everywhere, and Jess's mum is in danger.
There is much more to tell than this, but that would give the story away.