The most horrifying movie scene you will watch this year begins with a giant waterfall cascading into a lush green gorge. A Muzak cover of "Born Free" plays as smiling young women in matching satin gowns sway in the fall's mist. At the center of it all, two old Indonesian men—one squeezed into a ruffled, sequined turquoise evening gown like an overstuffed toy mermaid—raise their hands to the sky. In another context, it might be a cultural curiosity, or even make for a good laugh. In The Act of Killing, it's truly stomach churning.  

Some backstory: In 1965, a violent military coup in Indonesia overthrew the 22-year presidency of Sukarno (just Sukarno; like Cher), and ultimately led to the rise of Suharto (also like Cher; this trend was broken by his successor, Bacharuddin Jusuf Habibie), who would go on to lead the country in a repressive dictatorship for the next 31 years. His reign kicked off with a five-month anti-communist purge, which saw some 500,000 people killed. 

In the North Sumatra capital of Merdan, the job of slaughtering accused communists (and a good number of local ethnic Chinese) was given to a man named Anwar Congo and his pals—local thugs who previously spent their time hanging out in movie theaters and selling tickets on the black market. 

The Act of Killing picks up almost 50 years later. Today, Congo is a local hero, still living large on his reputation as a ruthless killer. Though Suharto eventually resigned in the late '90s, his crimes went unchallenged and unpunished. Congo—an otherwise amiable old man who enjoys singing, dancing and playing with his grandsons—speaks openly, proudly and in sickening detail about the hundreds of lives he took, strangling men with nothing but a piece of wire. He hobnobs with senior politicians and is a guest on a state-run television talk show, on which the young anchor gushes over Congo's movie-inspired killing methods and applauds him for his part in "exterminating communists."

History, as the saying goes, is written by the victors, and American director Joshua Oppenheimer's masterstroke is in allowing Congo and his cronies to go one step further. He asks them to make a film of their own, re-enacting their glory days from the death squad. They're only too happy to oblige.

This is how we end up panning across a scenic waterfall, with Congo and his right-hand-man—a pudgy dolt who inexplicably spends half of their film in garish drag—swaying serenely in the breeze. Two disheveled "communists" appear, remove their wire nooses, put a gold medal around Congo's neck, and thank him for executing them.

Elsewhere, the gangsters re-create their old interrogation and execution techniques in noirish homage to their beloved gangster movies, with Congo playing the part of communist captive. It is here that he finally starts to confront and question the atrocities he committed five decades ago. This should provide some small comfort to the audience, but it really doesn't. As Oppenheimer gently points out from behind the camera: "You know it's only a film. They knew they were being killed."

Like visiting Auschwitz or the Killing Fields, sitting through The Act of Killing is one of those wholly distressing experiences to which we submit ourselves in an effort to comprehend the great atrocities of humanity, and memorialize the lives left in their wake. It is also a spectacular, completely gripping piece of documentary filmmaking—but it's hard to appreciate the mise-en-scène when you're watching an old man boast openly on film about the 14-year-old girls he raped while burning down their village. It will cost you two very difficult hours, and in return you'll probably cry and maybe hurl.

But of all the uncomfortable experiences you might endure in the name of understanding the evils of history, this film is arguably the kind that is most necessary. There was no justice served here, no war crimes tribunal, no great memorials. The bad guys won. Congo will live out his days perhaps a little more tortured by his actions, but ultimately a wealthy, revered and free man. If you don't already know this awful chapter in history, a few hours and your appetite are probably not a lot to sacrifice in exchange for keeping the story alive. It's the least—and, sadly, the most—you can do. 

Critic's Grade: A+

SEE IT: The Act of Killing opens Friday at Cinema 21.