Dr Richard Clay goes in search of what it is that has made us scribble and scratch mementoes of our lives for more than 30,000 years. From the prehistoric cave paintings of Burgundy in France, through gladiatorial fan worship in Roman Lyons to the messages left on the walls of Germany's Reichstag in 1945 by triumphant Soviet troops, time and again we have wanted to leave a permanent record of our existence for our descendants. And it may be that this is where what today we call art comes from - the humble scratch, graffiti.

After Tom Hurndall is shot in the head in Gaza, his parents Anthony and Jocelyn arrive in Israel wanting to know how it could have happened. They expect sympathy and cooperation from the Israeli authorities, but are instead met with an official explanation that fails to tally with any eye-witness accounts, and a wall of silence. When an Israeli army report attempts to whitewash the incident, the Hurndalls decide the only way to establish the truth is to launch their own investigation into the shooting, a process which brings them face to face with both the Open-Fire regulations of the Israeli army in Gaza, and the soldier who pulled the trigger.