Purged of the chief torturer - my friend and saviour

The Conservation Office at Angkor in Cambodia is where the best pieces of Khmer art from Angkor and surrounding sites have been stored by Unesco experts since the 1990s to prevent them from being looted. Wandering through its profusion of stone Buddhas and Hindu deities recently, I was riveted by an ancient five-headed naga , or snake deity, whose teeth and eyelids had recently been painted blood-red, and whose cobra hoods were also flecked with red. Nagas are integral to the art of Angkor and are normally regarded as benign; but this one looked distinctly sinister. The storekeeper showed me the date of accession, 1999. The unknown image had been part of the art collection of Ta Mok, a notorious commander of the Khmer Rouge, who had finally been arrested that year, after the death of Pol Pot. His collection - looted from various sites - was transferred to the Conservation Office. Whether Mok himself was responsible for the painted graffiti is not known, but he might have been. After all, Mao Zedong, who inspired the loathsome Khmer Rouge regime that ruled Cambodia from 1975-79, is also known as a considerable calligrapher.

François Bizot, a French scholar of Khmer Buddhism who married a Cambodian in the 1960s and who now holds a chair at the Sorbonne, came within a whisker of being one of Ta Mok's victims. Arrested in 1971 by Khmer Rouge soldiers in a village while researching religious rites, he was condemned by Mok as a CIA agent but was eventually released - the only western prisoner of the Khmer Rouge who lived to tell the tale. He owes his life, ironically, to the man who later became the chief torturer of the regime, alias Douch, who became persuaded of Bizot's innocence and in turn persuaded Pol Pot. (Douch, too, was finally arrested in 1999.) The first half of Bizot's gripping and complex book is the story of his incarceration in the jungle, much of the time shackled in chains, and his growing "friendship" - a word Bizot himself uses - with Douch. The second half jumps more than three years to April 1975, and describes the desperate situation of the French and foreign community in Phnom Penh after the Khmer Rouge takeover. Bizot, as a fluent Khmer speaker trusted to some extent by the Khmer Rouge, became an unofficial negotiator between the French consul and the regime as it forcibly evacuated the capital city. "The gate" of the book's title is the flimsy gate of the French embassy that separated their sanctuary from the living hell that was about to become the killing fields.

Bizot's task was to find foreigners and food in the destroyed city, to prevent the killing of the embassy's occupants, and to help get them out of Cambodia.

Those wrenching weeks are described with all the power of a superb thriller. But Bizot's earlier relationship with Douch is what makes The Gate unique. As with the red paint on the naga head, we become fascinated by the motives of this former mathematics teacher who turned to communism in the 1960s, an educated, conscientious young idealist who was brave enough to compel his superior Ta Mok to return the Swiss watch he had stolen from the condemned Bizot. In a long tête-a-tête about the revolution around the camp fire, after his release has been announced, Bizot provokes Douch to say, chillingly: "Comrade, it's better to have a sparsely populated Cambodia than a country full of incompetents!"

But in the end we never truly understand the torturer's mind. As Bizot describes in an extraordinary epilogue, in the late 1980s he visits Tuol Sleng, the former school in Phnom Penh turned into a devouring prison by Douch, and in 1999 he even corresponds at length with his arrested former captor, who carefully draws him a map of the former jungle prison camp so that Bizot can locate it and revisit the place where all but he were cudgelled to death by Douch's young guards. In the last words of this bitter book, which took almost three decades to produce and is justly called "an original classic" by John le Carré in a fine foreword, Bizot is able to write: "I am purged of my ghosts. I have emptied my memory. I close The Gate behind me. Puppets, hung clustered on a rail, dangle in the twilight, an offering to the wolves. I turn back one last time; on the other side of the grille, Douch has joined them."

Andrew Robinson is literary editor, The THES .