The collision team are called in to investigate a high-speed crash on the M4 which closes the motorway in both directions. With a driver under arrest and offering few clues, the answers could lie with a silent witness.

It's Friday night, and desperate 999 calls start flooding in to report a car travelling the wrong way along the motorway - but it still ends in tragedy. A motorbike and van collide in a busy city centre, but the story isn't as clear as it first seems.