The police may have a serial killer on their hands when they locate a second body in a site where the first victim was found several months before. The body was trussed up in the same way and had an identical burn mark on his back. There is no apparent link between the first victim, student Michael Haughty, and the second Jason Villers, a probationer at the police college. While the police find evidence that may solve the Otty case, they have nothing on the Villers murder. In a separate case, Harry is approached by the wife of an old university friend who asks him to perform a post-mortem on her husband who died in a fall. Harry finds that his old friend was injecting himself regularly. Leo's family is visiting for a few days but he's constantly being called into work.

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.