Selena Murphy is commuting home from her job in the city when she strikes up a conversation with a beautiful stranger in the next seat. The woman introduces herself as Martha and confesses that she's been stuck in an affair with her boss. Selena, in turn, confesses that she suspects her husband is sleeping with the nanny. When the train arrives at Selena's station, the two women part ways, presumably never to meet again. But days later, Selena's nanny disappears.