Return of the documentary, this time following the work of Derbyshire's forensic collision investigators as they use the latest technology and detective work to find out what went wrong and why. On the A57 in South Yorkshire, investigators are called to the scene after a speeding car knocks down an 81-year-old cyclist at a pedestrian crossing in a suspected hit-and-run. With news the cyclist has died, the investigation turns into a hunt for the killer