The Poison Tree is based on the dark and captivating book by Erin Kelly. Karen Clarke has been waiting nearly twelve years for her partner Rex to be released from prison and is looking forward to settling down to normal family life in their seaside cottage with their 11 year old daughter Alice. After she begins to receive some silent phone calls and anonymous text messages following his release, she can't get rid of the feeling that she and her family are being watched. The show flash backs to 1999, when Karen was a student and first met Rex and his sister Biba. It reveals Karen, Rex and Biba's shared history in the crumbling Highgate mansion that they shared and how events led up to two people being dead. With the threat to Karen drawing ever closer, gradually the pieces of the puzzle fall into place, and the truth of what happened on that fateful day is finally revealed.