The siblings' growing suspicions about Mark are fuelled by rising tensions in their own lives: Helen's focus on her high-powered job has led to a deep rift in her marriage, Jake's gambling debts have caused his wife and kids to leave him, and Natalie is lying to everybody about her secret relationship. Meanwhile Vivien's overly attentive neighbour, Mary, is simply trying to protect the family she has known for years. But in a few short weeks one of these people will be lying on the beach, terribly injured - perhaps dead. The question is, who?