Documentary series following the traffic police as they enforce the law on some of Britain's busiest and most dangerous roads. The cops are called to an extraordinary hit-and-run incident in Cardiff. It was caught on camera, and when they see the video they cannot believe their eyes. On a quiet Sunday morning in Swansea, another pedestrian is shockingly run over and left for dead. The episode was also captured on CCTV but the footage doesn't answer the vital question: who was the driver?