At first he feared that he would be caught and end up in front of a firing squad. He soon learnt that the security guards never took more than a cursory look. At weekends the Colonel took the bundles of notes to his dacha. There he would clamber under the floorboards, often scattering rats, spade in hands and hide the notes in a buried milk churn. Although he hoped that one day his files would expose the evils of the KGB, he could hardly have dreamt of the impact his archive would have when it was published in the West more than 15 years later.
Col Mitrokhin's researches led to the unmasking last week of Melita Norwood, the little old lady from Bexleyheath who was one of the KGB's biggest British catches. Tonight, more revelations are promised in BBC TV's The Spying Game. This week also sees the publication of the book on which the expose was based. The Mitrokhin Archive, The KGB in Europe and the West offers more revelations still. We now know that the KGB planned to disrupt the 1969 investiture of Prince Charles, maim the ballet dancer Rudolf Nureyev and arrest Cardinal Wojtyla, now Pope John Paul II.
Yet none of these plans was carried out. Paradoxically, the book shows the KGB, despite the hype, to have been curiously diffident and unimpressive.
The real revelation has been the light shone on the dark and secretive world of MI5 and MI6. The lifted stone revealed an arrogance and a lack of political nous, particularly in not consulting ministers before deciding not to prosecute.
Yet The Mitrokhin Archive, some 1,000 pages spanning the entire history of the Soviet regime from 1917 and into the reforms of the 1990s, reveals little of significance that was not already known. Historians knew that many Soviet agents in Britain and elsewhere had not been caught. Even the existence of agent Hola - Mrs Norwood - is not entirely new. She was almost caught when Special Branch rounded up a spy ring based in Woolwich Arsenal in 1938. And British and American security services knew in 1945 that there was a Soviet spy in Britain code-named Tina. The "Venona" decrypt of intercepted communications in September 1945 had revealed agent Tina was handing over information on atomic research to the Soviets. "Tina" appears to have been another one of Mrs Norwood's code-names.
The book does tell a good story about Venona itself. The decrypts were one of the greatest intelligence secrets of the Cold War. They were intercepts of high-grade diplomatic and intelligence communications. Some have been declassified only in recent years although they date back to the Second World War and early Cold War. Venona was such a closely guarded secret that President Truman was never informed, nor were more than a small number of the Attlee cabinet in Britain. Intelligence agency rivalries meant that even the CIA was not told until 1952. However, as the book confirms, Stalin knew from 1947. A Soviet agent, William Weisband, who worked for the US Signals intelligence agency, passed the secret to his Soviet controllers.
Some of the best stories in the book have little historical value but are often amusing accounts of bungling in the spy world. There are some surprising omissions. There is no mention by Mitrokhin of the Soviet agents David and Rosa. During the Spycatcher affair in the mid-1980s it was suggested that they were Lord Victor Rothschild and his wife Tess, both now dead. Both were close to members of the Cambridge spy ring. The identity of the two spies is perhaps the most interesting question that remains about the Cambridge spy ring. Yet there is not a word about them in the book.
What the book does demonstrate is that the Cambridge Five - Philby, Burgess, Maclean, Blunt and Cairncross - were the apex of Soviet intelligence success. There were plenty of other recruits, many of great importance. But none matched the importance of this group of men who worked themselves into key positions in the British establishment.
They were recruited when many in the West had high hopes of the new Soviet system but as the century progressed and it was exposed as a ruthless anti-democratic philosophy, the Soviets found it harder to attract ideologically motivated spies. Increasingly, their recruits were driven by personal grievance, greed or plain eccentricity. On television, Mrs Norwood turned in a brilliant performance as a biddy whose dedication to the cause had been unfazed by Stalin's purges, the 1956 Hungarian uprising or the invasion of Czechoslovakia in 1968.
But the urgent lesson of the Mitrokhin affair comes from the political row that has ensued. For two years, the renegade MI5 agent David Shayler has been claiming that the security service was a bunch of bunglers who were potentially out of control and dangerous. The events of the past eight days have helped his case.
We now know Col Mitrokhin contacted MI6 in Latvia in autumn 1992. In a well-executed MI6 operation, Col Mitrokhin, his wife and archive were spirited out of Russia into Britain. MI6 seem to have conducted this operation without informing the Foreign Secretary. Then Mitrokhin was handed over to MI5 who debriefed him and translated the archives. The task was to identify Soviet spies and security failures. This the agency seems to have done at a leisurely pace without much interest in or hope of prosecution.
But senior MI6 and MI5 officers must have discussed the publicity value of their coup. It was decided to give the material exclusively to the intelligence historian Professor Christopher Andrew. This is an old ruse: get the defector, follow up the leads and have a book published. The book gives the defector some cash and British intelligence good publicity. Professor Andrew is used to the routine. He wrote three books with Oleg Gordievsky, the senior KGB officer who defected to Britain in the 1980s.
MI6, at least, decided to cover itself by telling its minister in 1996, but from Foreign Secretary Sir Malcolm Rifkind's recent account, it seemed like a fairly low-key briefing.
You can almost hear the conversation. MI6 chief: "Minister, now we have this KGB chap - defector, you know - brought out some of the KGB files a while ago - mostly historical, but one or two things of use. Like to hand the chap over to Andrew at Cambridge - you know Chris - safe pair of hands. Cambridge is a bit different these days - ha. The Prof will do the book and we'll get some glowing publicity for the service. You know the media, love a spy story. OK? Good."
However, for reasons that remain unexplained, MI5 does not seem to have told its political line manager, the Home Secretary - not, at least, for five years.
In the hot seat as director-general of MI5 is Stephen Lander. Much of the debriefing of Col Mitrokhin was conducted under his predecessor, Stella Rimington, the agency's first woman head, but Lander, 51, her deputy, took over in April 1996 and will take the brunt of criticism. Unlike his predecessor, he is not seen as a skilful Whitehall operator. His talents lie as a counter-terrorism expert.
Jack Straw, Home Secretary, has asked the House of Commons Intelligence and Security Committee, chaired by Tom King, to investigate the Mitrokhin affair. The heads of both MI5 and MI6 will be among those who give evidence. It will probably make better reading than the Mitrokhin book. But you can bet it will be kept secret.