The Truth Teller tells, but not always the truth.

Carol (Amanda Redman) and her husband Mark (Adrian Dunbar) are about to celebrate their 20th anniversary. At first glance, their life is idyllic. Mark owns his own car import business and they maintain a close relationship with Carol's sister Julie (Saskia Reeves) and her husband Trevor (Adam Kotz), who also works for Mark.
When Carol receives an anonymous email regarding an affair between her husband and his secretary, the seeds of doubt take root. Torn between her loyalty to her husband and her desire for the truth, she decides to confront the mistress and discovers her murdered. Forced to question how well she really knows her husband, her mind is overcome with suspicion. Has Mark not only betrayed her with another woman, but is the man she loves a killer?

Events quickly spiral out of control as Carol is further manipulated by anonymous emails and launches a search for answers that leads her to uncover truths about those closest to her that someone may not want her to know.