Olivia Carter, a seemingly ordinary commuter, witnesses a disturbing encounter on a train: two unsettling men engage in unsettling flirtations with a group of teenage girls. Though shaken, Olivia hesitates to intervene. The next day, one of the girls, Anna Ballard, vanishes without a trace.

Haunted by guilt and fear, Olivia begins to receive chilling anonymous threats. As the police investigation stalls, Olivia delves deeper into Anna's world, uncovering hidden secrets among her friends and family.

She discovers that the events on the train were more than just a random encounter. A web of lies and deceit, stemming from a tragic accident years prior, connects the men to Anna's disappearance.

Facing escalating threats and the constant fear of being watched, Olivia must confront her fears and use her newfound knowledge to bring the perpetrators to justice, while grappling with the guilt of her inaction and the lingering trauma of the experience.