Three years ago Alexander Blackman, better known as Marine A, became the first serving British soldier to be convicted of murder since the Second World War for the killing of a badly injured Taliban insurgent on the battlefield. He has now had his murder conviction reduced to manslaughter. Panorama re-examines the case to find out what really happened on that fateful day, six years ago.

Film-maker and anthropologist Chris Terrill has been following the case since the beginning, including meeting Blackman a number of times in prison. Chris has spent many years working with the Royal Marines and was embedded with Blackman's unit at the time the incident occurred in Afghanistan. He has tracked down some of the men who were with Blackman that day and they speak for the first time about the six-month tour that proceeded the killing and reveal disturbing new insights about the day itself. Questions are raised about whether there were other pressures on Blackman to act.

Using material Chris shot in Afghanistan when embedded with Blackman's unit, the film provides a stark sense of what it was like to be stationed in a 'kill or be killed' outpost set in what was then described as 'the most dangerous square mile in the world'. The new evidence is presented for the first time and raises concerns about the way our soldiers fight wars and apply ethics on the battlefield. The outcome of the Marine A case could well shape the way we fight our wars in the future.