A four-car crash and a multi-emergency response - the team must act quickly to piece it together. And after a single-vehicle crash on the M4 on Christmas Day, they are suspicious that there is more to it than meets the eye, and turn to science to get to the truth.

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.