''I have not seen a film as powerful, surreal and frightening in at least a decade...it is unprecedented in the history of cinema,'' remarked Werner Herzog, his words providing a suitably stark pull- quote for the poster. So profound was the film's impact upon Herzog that, alongside fellow film-maker Errol Morris, he felt compelled to sign on as executive producer following last year's preview screening. Yet such endorsement transcends the usual pre-release hyperbole, in that The Act of Killing truly is a film like no other. Indeed, it almost defies categorisation: it asks its audience to abandon judgement, while stretching the phrase 'a difficult watch' into a darkly comic understatement. But it's a story that must be told. As a result of a 1965 failed military coup in Indonesia, up to 2.5 million citizens were slaughtered on suspicion of being "communists". Those who were spared death were forced into prisons and concentration camps. But the reason why this particular page in the history book has yet to be turned is due to the fact that not only did the killers (many of whom belonged to state-sanctioned 'death squads') escape justice but, having been slotted into the current government, they are free to crow about their crimes and live the proud lives of protected men. Director Joshua Oppenheimer originally sought to make a film about those living in the aftermath, but after finding himself on the wrong side of the authorities, and realising that, even now, many of the victims' families were simply too scared to speak out, he followed the suggestion of one of his subjects and decided to interview the perpetrators instead. Yet almost fifty years after the massacre, he finds that time has not weighed heavy on these men's minds. They may be grey-haired and surprisingly gentle, but they haven't quite shed the skin of their former selves. Within minutes of meeting them we find that they are disconcertingly unashamed of their past, gleefully unrepentant. But given their involvement in their country's darkest chapter, is there not, asks Oppenheimer, even a scintilla of sorrow? They barely give the question a moment's thought; instead they boast about how many people they have killed (often revelling in gruesome detail) and speak of the purge in an eerily nostalgic manner. Perhaps giving credence to the belief that history is written by the winners, Oppenheimer asked the men - militia, mobsters and murderers all- to express themselves ''in any way they wished''. Their response left the director speechless. For they chose to re-enact their crimes in homage to their favourite Hollywood films. In theory, the plot sounds like a tar-black comedy, nothing short of the most offensive and exploitative film ever conceived. In reality, it is a deeply disturbing, often surreal but always compelling study into the banality of evil. Not a drop of blood is shown in the film and yet death lingers in every frame. When you leave the cinema, the world will look like a very different place indeed. Contains Spoilers.