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On Selena Murphy's train home from work, a mysterious woman named Martha strikes up a conversation and shares a confession: she's having an affair with her boss. In turn, Selena shares her own secret: she suspects her husband is sleeping with the nanny. At Selena's station the two women part, and Selena never expects to see her again. Until she receives a message. It's Martha, but Selena never gave Martha her number. She brushes the message off - until days later her nanny goes missing, and Selena begins to wonder if it's all connected. Who is Martha, really? And what does she want with Selena?