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In a quiet housing estate in Newcastle, shots are fired through a patio window in the dead of night. One of the residents is injured, but neither she nor her boyfriend saw anything. With the neighbours too scared to talk, it is the job of crime scene manager Alan Sayers to gather the vital pieces of forensic evidence which could catch the shooter; 'we call it the Harvest - if you don’t get the Harvest, you don’t get the results'. Samples of the broken glass are taken in case they can be forensically linked to any shards of glass found on a suspect’s clothing, the remnants of the shotgun cartridges are taken for specialist analysis by a ballistics expert, and a sweep of the wider area reveals three unspent shotgun cartridges which may have been dropped by someone fleeing from the scene. These are fast-tracked to the lab to be examined by forensic scientist Sally Agnew with the hope of identifying a suspect: 'you have to swab them enough to get the most recent handler’s DNA but not so heavily that you pick up everyone’s DNA who has ever handled it'.

DNA found on the unspent cartridges confirms that their prime suspect has handled them, but not that he was the one who shot the gun. One of the country’s leading ballistics experts, Andre Horne, reveals the inner workings of a gun cartridge as he attempts to find further connections between the remnants at the scene and the unspent cartridges. But it is a surprise discovery made when the suspect is searched in custody that could provide the final clue they need to link him to the scene – a small piece of glass is found in his sock which could be from the shattered patio window at the crime scene. At the lab the refractive index of the glass found on the suspect is compared with that of the same taken by the CSI from the window. If the two match, the suspect will have some very difficult questions to answer.

The other side of the city, a dad who is paying a visit to his son discovers him dead and his flat covered in blood. It is the job of crime scene manager Kathryn Bolam to help the detectives determine whether or not it is a murder. The position of the body and abundance of blood lead Kathryn to lean towards foul play, "you've got a blood-stained knife on a computer desk, you've got blood trailing outside, you've got blood in every room in this flat. My gut says that it's something sinister and that it is a murder". Concerns deepen when a witness reports seeing a man seen near the dead man’s address with blood stains on his shirt who confessed he had 'done something naughty'. At the crime scene, Kathryn suspects the fatal wound may be on the front of the body but has to make the difficult decision not to move him – to do so too early in their examination of the scene would risk disturbing other key evidence. Forensic pathologist Nigel Cooper is called in to observe the body in situ before it is brought to him at the mortuary – it will be his role to determine whether it’s a murder or not; 'if there’s a body at a scene, what I really want is to move that body to the mortuary but I have to restrain that instinct as often the information that actually catches the killer is at the scene'. Ultimately it is the forensic post-mortem that will have to determine if a cause of death can be identified. Nigel Cooper explains, 'the police like black and white answers – we all do – and at the end of the post-mortem, what the police want the pathologist to say is 'this is a murder' or 'this is not a murder'. But until you get the body back to the mortuary, you don’t have all the answers'.