Silent Witness - Season 6

Sam Ryan assists the police in the investigation of a multi-vehicle collision that results in the death of 11 people. She is joined by two new colleagues: Dr. Leo Dalton, an experienced pathologist from Sheffield and Dr. Harry Cunningham. As they try to determine the causes of the accident, they find a severed arm that does not belong to any of the victims. They also have to deal with DCI Carol Deacon whose role in the investigation is not quite clear to them. Sam and Leo have somewhat different approaches to their job, particularly in how they deal with the police. Harry performs his first autopsy on a child.

The mystery of the severed arm is partly solved when the police discover a body floating in the river with a missing arm. The pathologists continue their investigation with little help from the police and DCI Deacon in particular. When she reveals to Tom that two of the men in the accident were undercover policemen, he realizes that Sam was correct and that he is being manipulated. The police refuse to tell them what is going on and Sam's interference has serious consequences. Harry continues to try and find a balance between detachment and compassion as one of the survivors of the crash seeks his assistance.

The police investigate an apparent break-in that results in the death of Derek Irons, his wife and youngest daughter. Two older children survived the attack but one is in serious condition and in a coma. Irons was a ruthless businessman who targeted his rivals' customers resulting in regular confrontations. Irons was stabbed but his wife and daughter were shot. Sam is reunited with an old friend from Northern Ireland, Det. Supt. Tony Ashton who is the senior investigative officer on the case. Harry assists the police with the death of an old woman who is believed to have fallen down a staircase. His post-mortem reveals evidence that is not consistent with a fall.

The police continue their investigation into the deaths of several members of the Irons family but based on the forensic evidence, they now believe he may have been killed elsewhere and then transported to his home. When one of Irons' two surviving children is killed, they look to his circle of friends and family for a possible motive. Thanks to Harry's information, the police investigate further into the death of an elderly woman who supposedly fell down the stairs in her home. Supt. Tony Ashton clearly wants to renew his friendship with Sam but she isn't so sure. Leo's home life is in turmoil and it's been affecting his work.

When a decomposed body is found hidden in the wall of an old factory building, Sam Ryan concludes he was murdered. He is eventually identified by his ex-girlfriend Selina Thomson as Marcus Saul, who had simply disappeared one day some seven years earlier. There has also been a break-in on campus and drugs have been stolen. The Dean asks Leo Dalton to speak to some of the students on the issue, but he's not quite sure what he can do.

With the death of a student from a drug overdose, the university must face the wrath of the girl's father, a major benefactor. The autopsy reveals that not only had she taken drugs but also that she had sexual relations with three different men in the previous 48 hours. Although the police have a suspect in the murder of Marcus Saul, new information suggests that the identity of the killer may lie in a completely different direction.

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.

When a second police probationer dies at the police college, Sam concludes that they may be dealing with a copycat murder rather than a serial killer. The investigating officer thinks that a fellow probationer may have been responsible for the original murder but the college's commandant, whose son is a student there and soon to be a police constable, would like the reputation of the college protected and the whole thing swept under the carpet. Harry learns that his late friend's wife was prescribing the drugs he was taking and is not quite sure what to do with the information. Leo's relationship with his wife and daughter is stretched thin when he spends most of his time at work and they are left on their own.