Simon and Jane and Barney come to Trewissick, in Cornwall, enlisted by their mysterious Great-Uncle Merry to help rescue a priceless golden grail stolen by the forces of evil - the Dark. They are not at first aware of the strange powers of another boy brought to help, Will Stanton. Nor do they realize the sinister significance of the Greenwitch, a traditional image of leaves and branches that for centuries the women of Trewissick have made, each spring, and cast into the sea for good luck in fishing and harvest. But this is the time of the Greenwitch's making, a night-long ceremony that Jane is allowed to witness, in mingled fear and pity, and it shapes all the events that follow.
In a series of disturbing and sometimes dangerous incidents, the children discover the whereabouts of the stolen grail, the Greenwitch - possessed of a dreadful, vengeful power - is called from the ocean depths by the Dark, and its Wild Magic is loosed over the land. Yet, because of Jane's pity, the Greenwitch makes to her a strange gift, for a time at least, will keep the Dark from rising.
Greenwitch is the third of five books in Susan Cooper's memorable sequence, The Dark is Rising. Its central theme, supported by legends from the myth-haunted West Country of England, is that of unpredictable Wild Magic of the earth, "a force neither of the Dark nor the Light nor of men." - Book Jacket