It didn't happen one night in Leningrad in 1945

When two humans of genius meet, the result is often fateful for both: Goethe and Schiller drawn irresistibly together by an indefinable magnetism; Wagner's and Nietzsche's lives star-crossed from the start. In this strange, gripping, and often highly speculative book, half novel, half history, very ably translated by Antony Wood, György Dalos describes what the publishers claim to be "the most extraordinary encounter in the history of 20th-century literature", that between Anna Akhmatova and Isaiah Berlin in Leningrad in 1945.

The reader may at first feel sceptical. Yet consider. Here was a woman poet of 56 who, by 1911, had become a leading figure in the pre war flowering of Russian poetry now known as the "Silver Age". An intimate friend of Pasternak's, she was a writer whom Chukovsky called "Pushkin's great successor", and literary textbooks refer to her as "the greatest poetess of the Russian tongue". She personally educated and inspired later poets such as Brodsky; and, on the award of her honorary doctorate at Oxford in 1965, was hailed as the "matriarch of Russia's poets" and "the Russian Sappho".

Meeting her was the 36-year-old Isaiah Berlin, already legendary, then first secretary to the British Embassy in Moscow. For him, as he has described it, this was like being suddenly ushered into the presence of a historic personage as remote and inaccessible in time as Christina Rossetti. Here was the man who was to become the leading philosopher of liberalism in the 20th century and the custodian and continuator of the values of the 19th-century radical Russian intelligentsia in exile; a figure who won the trust and respect of men of power such as Churchill, Weizmann and Kennedy; and a thinker who, on his death, was acclaimed as one of the supreme intelligences of his time.

This is the terribly touching story of a single night and of its fateful consequences. In November 1945, in Akhmatova's apartment in Fontanny Dom, the old Sheremetev Palace steeped in historic associations, the two sat down at nine in the evening and talked for 12 intense hours without a break. Berlin was the first westerner she had met for 34 years. Like a skilful dramatist, Dalos divides up the course of this meeting into acts and scenes. In Personal Impressions , on which Dalos leans heavily, Berlin has given his own unforgettable account of that evening and of what Akhmatova said to him. She ranged freely and openly over her entire life, its unspeakable tragedy and suffering, its every intimate detail, and utterly bared her soul to him. Yet in his essay Berlin remains tantalisingly reticent about his side of the conversation.

This leaves Dalos with no alternative but speculation: Berlin must have cast aside the persona of the cool British diplomat to open a door on his innermost being. At its core was the young Jew from Riga who had carried the Russian language and literature with him into exile, and there preserved an alternative Russia of the mind. In any event, whatever he said moved Akhmatova so profoundly that, in a stanza of "Poem without a Hero", dashed off feverishly after his departure, she chose him as her "Guest from the Future". This mysterious figure seems to have haunted Akhmatova's imagination for many years before their meeting and she came to see it as a kind of premonition of Berlin's arrival, indeed as a symbol of Berlin himself. Henceforth, she was possessed and saw his hand in everything: the award of the Taormina prize in 1964; the visit of English undergraduates in 1954, which caused her and Mikhail Zoshchenko such serious difficulties; and of course her Oxford doctorate. She thought of him literally every day. As an old lady she would stand for hours at the window of her remote "hut" on the look-out for unannounced visitors. After her last meeting with him in the splendour of his Oxford mansion one year before her death, she penned the lines: "Not to a secret pavilion/ Does this flaming bridge lead:/ Him to a cage of gold,/ And her to a red scaffold."

It had been no normal love affair. There was no physical contact. This must be one of the purest encounters between two human personalities on record. Two extraordinary minds seem for a moment to have engaged perfectly together to drive each other up to ever greater heights of mutual love and understanding. Indeed, it may stand as a kind of ne plus ultra , the very Platonic idea of human communication. Dalos even suggests that after this first encounter Akhmatova seriously entertained the thought that she would one day marry Berlin. Perhaps Berlin sensed or even knew this, and it could explain why he delayed any attempt to visit her again until after his marriage in 1956, and also why she declined to see him on that occasion.

For Berlin it was "the most memorable encounter of my life". It altered the course of his career for ever, and he was later to tell Martin Malia of the University of California: "Akhmatova and Pasternak gave me back my homeland." It called forth the most sustained and unanswerable defence of human dignity, freedom and liberal values in the darkest of dark centuries,and gave us works on Belinsky, Herzen, Tolstoy and Two Concepts of Liberty . For Akhmatova it was the pivotal point of the last phase of her creative activity, and possibly of her entire life, and it led to the flowering of some of the finest lyrical poems of the 20th century or any other time.

Yet it had other, very tragic consequences. After Berlin's visit, Akhmatova's flat was bugged by the KGB, who placed a number of informers close to her. Stalin himself became personally involved. "So the nun is receiving foreign spies, is she?" His constant personal interest in her life and fate is very odd and forms a constant theme running through this book. Was it envy on the part of Stalin, the erstwhile poet? Was it Akhmatova's capacity to fill entire stadia for her public readings and to command standing ovations of a quarter of an hour? There is something antique and Neronian about the intense and nerve-racking relationship that existed between outstanding poets and artists and the supreme ruler of the land. In 1946, Stalin's cultural watchdog, Andrey Zhdanov, secured a resolution condemning the journals Zvezda and Leningrad for publishing her poems. She and Zoshchenko were expelled from the Union of Soviet Writers and their works banned. In September her former husband Punin was arrested and her own life was made systematically unbearable. Then, in 1949, her son, Lev Gumilyov was sentenced to ten years' labour in Siberia. In the Gulag, he was subjected to questioning about Berlin.

Ever after, Berlin reproached himself bitterly for Akhmatova's fate. And Akhmatova herself was convinced that their meeting was to blame for what befell her and those she loved. Indeed, when in 1965 in Oxford they met again for the first and last time since 1946, she told him that together they had started the cold war. Berlin was more than sceptical but remained silent. Dalos, however, is not so sure. In a chaotic and unpredictable world governed by terror, violence, paranoia and the arbitrary will of an absolute dictator, fantasy and reality coalesce and anything is possible. That the first secretary of the British Embassy should be walking openly around Leningrad with prime minister Churchill's son and visiting leading dissident Russian writers was bound to cause commotion in the Kremlin. Akhmatova gave poetic expression to her apocalyptic belief in the "Third and Last" dedication to "Poem without a Hero" - "He will not be a beloved husband to me/ But what we accomplish, he and I,/ Will disturb the Twentieth Century." Perhaps in a sense not fully intended by Akhmatova their work is destined to disturb the 21st century, too.

Roger Hausheer is lecturer in German, University of Bradford.