THE SINNER follows a young mother (Jessica Biel) who, when on a day trip with her husband (Christopher Abbott) and son, commits a startling act of violence and, to her horror, has no idea why. The event launches an inverted and utterly surprising crime thriller whose driving force is not the "who" or the "what" -- but the "why." When an investigator (Bill Pullman) finds himself obsessed with uncovering the woman's buried motive, together they travel a harrowing journey into the depths of her psyche and the violent secrets hidden in her past.