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 one 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.

At Hillsdon Army base,a training camp for soldiers fighting in Afghanistan,young soldier Mark Blakefield is still recovering from the death in action of his friend Danny Ferris and argues with Lieutenant Lockfield,who was having an affair with Danny's girl-friend 'Cats' Felton,a fact which Lockfield's wife Claire has also discovered. Cats' body is pulled from the river some time after she was talking to the lieutenant and Nikki suspects foul play. Then Lockfield is shot,an apparent suicide,but Harry,accompanying Inspector Suzy Harte to the base,finds from the post mortem result that the man was murdered.

Nikki links Cats to the Lockfords via her phone and,at her flat,sees photos of her with her boy-friend Danny,whose father identifies her corpse. The police catch a man skulking at her flat,Dennis Croft,a former soldier cashiered for drug peddling who says there was a cover-up over Danny's death. At the request of Danny's parents a further post mortem is carried out,ultimately proving that he was killed by 'friendly fire'.Consequently it transpires that Lockford was not seeing Cats for sex but seeing her right financially. CCTV evidence shows that Cats' death was due to a bizarre accident but who killed the young lieutenant and why?