The team investigates the derailment of a train caused by someone leaving Maj. Mark Wiltshire's Land Rover on the tracks. The army is suspected but Wiltshire was in the officers mess at the time. The team have yet to identify the 21st victim, or determine if that person was on the train or in the car. Working with a survivor's group, they try to re-create where everyone was seated just prior to the crash occurring. When of the survivors commits suicide, the crash claims its 22nd victim.
The investigation continues and one of the survivors remembers a vital piece of information. It becomes evident that the identification of body 21 may also lead to the cause of the crash. In the end, the solution rests on discovering a an unknown romantic affair and marital betrayal.