Confessions on the 7:45

Everyone has a secret. Who would you trust with yours ?
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. I’d love to continue our conversation. Can we get together ? It’s Martha, by the way. From the train.
 
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 ?