Theo Conroy (Bacon) is a successful middle-aged man whose marriage to his much younger actress wife, Susanna (Seyfried) is shredding at the seams, frayed by her secretiveness, his jealousy, and the shadow of his past. In an effort to repair their relationship, Theo and Susanna book a vacation at a stunning, remote modern home in the Welsh countryside for themselves and their six-year-old daughter, Ella (Avery Essex). What at first seems like a perfect retreat distorts into a perfect nightmare when Theo's grasp on reality begins to unravel and he suspects that a sinister force within the house knows more than he or Susanna have revealed, even to each other.