On a recent trip to Cambodia, I met a genocide-denier. He was a middle-aged New Zealander who has lived for the past dozen years outside Siem Reap, the town near the temple-city of Angkor. The area has long been a stronghold for remnants of the Khmer Rouge, the fanatical Communist army whose rule over Cambodia from 1975 to 1979 cost somewhere between one and two million lives, a surreal total in a nation of just 10 million. Victims were mainly bludgeoned to death and tossed into mass graves.

The New Zealander begged to differ. No more than 80,000 Cambodians "got a knock on the head with a hoe," as he put it with casual dispassion. The rest died of starvation or disease. The Khmer Rouge were "nasty" and inept, but not genocidal. They'd also been disciplined and forthright and presented a corruption-free vision of their nation. Things had simply gone terribly wrong.

François Bizot would probably find the New Zealander's views appalling. His disgust would lie both with the substance and tone of the denial. The Gate, Bizot's memoir of his experiences in Cambodia during the Khmer Rouge era, is passionate and intense. Though he waited 30 years to write the book, it is still done in "hopelessness" and with a "bitterness that knows no limits."

On October 10, 1971, Bizot, a young French ethnologist "drawn to the mysteries of the Far East," was conducting research at a monastery north of Phnom Penh. His small travelling party was detained by the guerrillas dubbed the Khmer Rouge, or Red–i.e. Communist–Khmers. The group already had a reputation in the countryside for extreme discipline and cruelty.

Accused of being a CIA spy, Bizot and his assistants, Lay and Son, were brought to a detention camp for interrogation. For three months, the presumed imperialist agent was chained and starved by the rebels. Around him, fellow prisoners of lesser stature disappeared into the bush with their captors, never to return.

Bizot's interrogator was a former math teacher known as Ta Douch. Because the Frenchman could speak the Khmer language, and understood Cambodian social decorum, the two men sparred verbally about everything from politics to camp conditions.

The stakes, Bizot learned later, couldn't have been higher; Douch was under orders to find any excuse to execute the Westerner.

In the end, Douch supported Bizot's release. Putting his own career at risk, the Khmer cadre virtually escorted the Frenchman to freedom. The fact that the same cadre later oversaw the atrocities at the notorious S-21 camp in the Phnom Penh suburb of Tuol Sleng, where 17,000 men, women and children were tortured and then taken away for execution, is mentioned only towards the end, when the author interviews Douch in prison in 2000. Curiously, little is made of the reunion, aside from an admission by Bizot that he felt "brutally unable to equate the vision of [this] loathsome executioner with the image of my liberator."

The use of the adverb "brutally" is telling. Despite the passage of time, The Gate reads like a fresh report from a battlefield. An obviously moral and humane person, Bizot cannot be reconciled to what he has witnessed. He now "distrusts man" and believes "only in things"; there is a "diabolical presence" buried in humans, especially where Utopian schemes are allowed to flourish.

Such emotional extravagence can be stirring. It can also make for a disjointed and even elliptical narrative. Bizot, or his editor, seems aware of this, and provides a chronology of events for those less-than-familiar with recent Cambodian history.

The second half of the book tells a better known story. It is now April, 1975, and the Khmer Rouge are marching on Phnom Penh. Within a few weeks the guerrillas will have emptied the capital of its citizens, part of its ludicrous "back to the land" ideology. Bizot is still in the country, collecting and studying Buddhist manuscripts, though he has wisely sent his family to France.

Most Westerners in the capital, along with thousands of locals, seek shelter within the grounds of the French embassy. Once again, Bizot's skills cast him in a leadership role, and he is soon negotiating with the conquering army. He also winds up a kind of "gatekeeper," an intermediary for various desperate pleas for sanctuary. In a series of gripping chapters, The Gate plays out a morality tale worthy of Joseph Conrad or Graham Greene.

Who will be turned over for likely execution? Who will be provided false documents to offer a chance at escape?

There is the elderly member of the Cambodian royal family who appears at the embassy gate wearing his Chevalier de la Legion d'Honneur medal. "I am Prince Sisowath," he tells Bizot. "I have the honour to request political asylum from France." To which the helpless gatekeeper can only reply: "Excellency, do you have a French passport?"

Likewise, the elegant young woman stepping out of a Mercedes with a baby in her arms. She is the wife of a politician who the Khmer Rouge have already tossed into a well. Once it becomes obvious she cannot enter, the woman attempts to fling her child over The Gate at Bizot. At a loss, he literally covers his eyes with his hands to block out reality. When he opens them, she has climbed back into her car with her baby, resigned to her "dreadful fate."

Being a witness to such scenes will mark even the bravest individual. Bizot writes more like a survivor than a chronicler, his eyes open to the horrors, his heart still unable to accept certain truths. (A detail about the fate of his assistants, learned only in recent years, nearly chokes the narrative in its despair.) His trauma is profound, and by rendering that condition so honestly he lends The Gate a grave dignity. The book is distressing and, perhaps surprisingly for the current chair of Southeast Asian Buddhism at the Sorbonne, entirely unable to let go of the suffering. It will certainly be hard to forget.