In the final episode of the series following the work of Gwent Police's Collision Investigation Unit, the team are alerted to a serious crash by the latest in-car technology, and turn to 3D laser scanning to help establish the facts of what happened. And after a van crashes into a bus shelter, they must find and interpret the evidence to solve the mystery of why the driver lost control.

The Crash Detectives are called in to investigate a hit-and-run that leaves a cyclist fighting for her life, while mystery surrounds the death of a delivery driver on a busy dual carriageway.

When calls start coming in to report debris on the M4 motorway, police discover that a father and son have been in a serious crash. As traffic chaos builds, the pressure's on for the collision team to find out why the car left the road.