"The Girl on the Train" is a psychological thriller novel by Paula Hawkins that follows the story of Rachel Watson, a woman who becomes entangled in a missing person investigation. Rachel, who struggles with alcoholism and personal issues, observes the lives of others from her train window, particularly a couple she idealizes. When the woman she has been watching goes missing, Rachel finds herself drawn into the mystery. As she becomes involved in the investigation, secrets are revealed, and her own past begins to unravel, leading to unexpected twists and turns in the narrative.