Billy the kid, the most notorious figure of the wild west, is on trial again... this time the world's three leading experts, re-examining old evidence and pondering letters from Billy the Kid that have never been seen before, try to determine just how guilty he was, and whether the then governor of New Mexico, Lew Wallace, really had promised Billy a pardon, and if so, why? Was Billy the Kid the psychopathic killer history says he is or was he completely different from the legend that has built up around him?

• Sam Heughan