When Michael Callaghan, the charismatic lecturer with whom Cambridge student Ophelia Marsh is having an affair disappears, she tracks him down to Ireland to discover that his wife has died. Learning that Ophelia is pregnant, Michael invites her to stay at his family home, where she begins to feel haunted, perhaps by more than just her guilt.

With Michael called back to Cambridge for a mysterious family emergency, Ophelia is left in Ireland to contend with the disturbing intrusions of Roisin's mother Mary. Isolated in the cavernous old house, Ophelia's curiosity about Roisin mingles with her paranoia, and her enquiries among the locals into the mysteries of Knockdara begin to take her down a darker path.

As her obsession with Roisin deepens, Ophelia begins to fear that Michael is not telling her everything, and she sees signs of the woman wherever she goes. Ruth's investigations in Cambridge reach Ireland with the arrival of a mysterious stranger, and Ophelia has to finally face up to whether she is losing her mind as Michael claims, or if something terrible, as she suspects, is afoot.

As the truth about Roisin finally breaks through the web of lies and deceit which has been spun between Cambridge and Ireland, Ophelia tries desperately to escape, while the architects of this manipulation attempt to cover their tracks. With Ophelia trapped and cornered in the House at Knockdara, she must finally find the courage to face up to those who have sought to destroy her.