On Selena Murphys train home from work, a mysterious woman named Martha strikes up a conversation and shares a confession: shes having an affair with her boss. In turn, Selena shares her own secret: she suspects her husband is sleeping with the nanny. At Selenas station the two women part, and Selena never expects to see her again. Until she receives a message. Its 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 its all connected. Who is Martha, really? And what does she want with Selena?