David Walker is a professor at the Faculty of Further Psychology at Helmstone University. One night, he rescues a drowning swimmer off Helmstone Pier. She is Helen Costelloe, a beautiful woman with a past that does not add up and a present that quickly gets entwined with Walker's - until one night he finds her gone.

Two years later, Walker receives an astonishing letter. Helen has inherited Malpas, a vast, crumbling Georgian house on one of Ireland's great rivers. She is the owner of thousands of acres of tangled forest, two ruined abbeys, a derelict private gasworks. Maplas, its estate and its inhabitants are a world of their own. A beautiful world.

But a world with chilling secrets, where madness stalks the riverside oak-woods.

Helen's new husband has drowned himself in the river - the latest in a string of bizarre suicides stretching back eighty years. Darkness is rising around her. She needs company. More than that, she needs help.