'To keep you is no benefit, to destroy you is no loss.'

During the Khmer Rouge's four-year rule of terror, two million people, or one in every four, Cambodians, died. In describing one family's decades-long quest to learn their husband's and father's fate and the war crimes trial of Comrade Duch (pronounced 'Doyk'), who ran the notorious S-21 prison in Phnom Penh, When the Clouds Fell from the Sky illuminates the tragedy of a nation.

In 2012, Duch was sentenced to life imprisonment, having been found responsible for the deaths of more than 12,000 people. He was the first Khmer Rouge member to be jailed for crimes committed during Pol Pot's catastrophic 1975-9 rule during which millions were executed or died from starvation, illness and overwork as Cambodia underwent the most radical social transformation ever attempted. Designed to outdo even Mao's Great Leap Forward, it was an unparalleled disaster.

At the same time, the Khmer Rouge closed Cambodia's borders, barred all communication with the outside world and sought to turn the clock back to Year Zero. They outlawed religion, markets, money, education and even the concept of family.

The revolution soon imploded, driven to destruction by the incompetence and paranoia of the leadership. Yet instead of recognising their own failings, the leaders sought unseen enemies everywhere. In their pursuit of purity, they destroyed a nation.

Like hundreds of other returnees, when he returned in 1977 Ouk Ket was utterly unaware of the terrors being wrought in the revolution's name. Hundreds of thousands of other Cambodians perished in nearly 200 institutions like S-21.

To illustrate this era and its consequences, Robert Carmichael has woven together the stories of five people whose lives intersected to traumatic effect: Duch; Ket's daughter, Neary, who was just two when her father disappeared; Ouk Ket himself; Ket's French wife, Martine; and Ket's cousin, Sady, who never left Cambodia and still lives there today.

Through these personal stories, the author's own research, numerous interviews and mo