In 1992, an elderly, ill-dressed and inarticulate Russian named Vasili Mitrokhin walked unannounced into the newly opened American embassy in Riga carrying a duffel bag stuffed with papers. He was shown the door without a hearing. Next, he went to the equally new British embassy, where a young diplomat used her intuition and persuaded MI6 to take a closer look. Two years later, Mitrokhin, his totally unsuspecting wife (an ear, nose and throat doctor) and son (crippled with motor neurone disease) were smuggled across the Baltic and into Britain, along with the papers, which included summaries of numerous KGB documents. Over the rest of the decade, this enormous archive was studied, translated and partly published, with the Cambridge academic Christopher Andrew acting as handler and translator.

For over a decade as archivist of the KGB’s First Chief Directorate (which had responsibility for foreign affairs and counterintelligence), Mitrokhin had produced handwritten summaries of the files in his care. At weekends, in the solitude of the dacha to which he retreated ostensibly to hunt and fish, he had typed these up. His family, neighbours and colleagues were totally ignorant of what he had been doing. So too were his superiors. The bags of archivists were searched when they boarded the bus home from Yasenevo on the southern outskirts of Moscow, site of the KGB archive, but not their clothes. Mitrokhin secreted his notes inside his jacket and trousers.

MI6, faced with hundreds of thousands of notes in minute handwriting and cryptic typescripts, were incredulous that so much information and kompromat could have been smuggled out undetected. They were tempted to suspect that Mitrokhin’s material formed part of a hoax designed to throw Western intelligence services into disarray. But