Jenny Bowen is going home. Following an acrimonious split from her husband, she is quitting her job and moving back from London to the small town in the Scottish Highlands where she grew up. After tying up some last-minute loose ends, she rushes to Euston and boards the Caledonian Sleeper, headed north on a 500 mile, 10-hour journey through the night. 

As Jenny is getting settled in her cabin, she hears a commotion outside in the corridor. She sees a pale little girl of about eight years old being bundled into a cabin down the carriage, with a stressed-looking woman whom Jenny assumes to be her mother. A tall man glances at the pair as he passes them. Jenny thinks it's strange that he has no luggage. He gives her a look that makes her uneasy before opening the door of the cabin next to her.

When they reach their destination in the morning, Jenny discovers the woman dead in her cabin...but there's no sign of the little girl. The train company have no record of a child of that age being booked on the train, and CCTV shows the dead woman boarding alone.

Jenny starts to get settled in her new home. She tries to put the incident out of her head and tells herself that everyone else is right: she must have imagined the little girl, or it was a misunderstanding. But deep down, she knows that isn't the truth.