It is Midwinter's Eve, the night before Will's eleventh birthday. But there is an atmosphere of fear in the familiar countryside around him. Will is about to make a shocking discovery — that he is the last person to be born with the power of the Old Ones, and as a guardian of the Light he must begin a dangerous journey to vanquish the terrifyingly evil magic of the Dark.

In their desperate quest to find the grail, stolen by the forces of the Dark, the Drew children have returned to Trewissick. One night, in a strange ceremony, Jane watches the village women construct the Greenwitch: an ancient offering to the sea. Overcome by the Greenwitch's power, Jane makes a perilous wish — one that will tip the balance in the ongoing struggle between good and evil.