Happiness is relative. For single mother Natalie Greene, that relative is her stepsister, Isabel Walker, known as The Happiness Guru. But even with Isabel's guidance, Natalie can't control her recently re-triggered PTSD over her mother's death in a car crash years ago. The old dread. The nightmares. And that all-consuming, terrifying thought: I think I killed my mother. But did she? And if not, who did?