El equipo investiga el descarrilamiento de un tren causado por alguien que dejó el Land Rover del Mayor Mark Wiltshire en las vías. Se sospecha del ejército, pero Wiltshire estaba en la sala de oficiales en ese momento. El equipo aún no ha identificado a la víctima número 21, ni determinado si esa persona estaba en el tren o en el automóvil. Trabajando con un grupo de sobrevivientes, intentan recrear dónde estaba cada persona sentada justo antes de que ocurriera el accidente. Cuando uno de los sobrevivientes se suicida, el accidente cobra su víctima número 22.

La investigación continúa y uno de los sobrevivientes recuerda una pieza de información vital. Se vuelve evidente que la identificación del cuerpo 21 también puede llevar a la causa del accidente. Al final, la solución radica en descubrir una relación romántica desconocida y una traición matrimonial.

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?