Every contact leaves a trace. The killer will leave something at the scene of the crime - hairs, fibers or they will take something away with them, such as the soil on their shoes. This program examines four cases all concerned with the interpretation of evidence.

The key piece of evidence that detectives are always desperate to find is the murder weapon, which can hold vital clues that will expose the killer. This episode looks at murder cases with unusual weapons

The first episode looks at the difficulty of identifying the body in a murder case. The question of identity is a crucial start to the investigation. From charred bones to bodies completely dissolved in acid, with each horrific new case science has had to adapt to identify both the victim and the murderer. Investigating four breakthrough cases, Gabriel reveals the scientific innovations that tipped the scales of justice in favour of the detective - and caught the killers.