The long voyage of Russia's unforgetting prophet ended not with a bang but a chat-show. Christopher Hope reports
D M Thomas argues that Russian writers were regarded by their many readers as a better form of government. They saw further and lied less than the usual tyrants. Pushkin, their hero, saw the Russian night as crowded with demons; Dostoevsky brought these to life in The Possessed. Come the Revolution, and the Bolsheviks set about creating homo Soveticus. Communism called itself a science, but was really a perverted religion that believed in engineering the souls of men, and holy homicide. So, in 1918, the demons stepped out of nightmare into government.

The poles of Solzhenitsyn's life are the prison yard and the pulpit. It fell to him to enter the inferno and to report what he had seen there. He began as a young revolutionary, worshipping Lenin: and ended up an Old Believer, sympathetic to the schismatic Russian church. Thomas shows the essence of Solzhenitsyn's life is confinement, whether punitive or voluntary. He is prisoner and monk: his natural habitat is the cell.

Alexander Solzhenitsyn was born on 11 December 1918 into a Russia torn by civil war. Soon afterwards, his father died in a shooting accident. A poor boy in a provincial town in the Caucasus, Solzhenitsyn went on to study mathematics at the University of Rostov. Then came the war with Germany.

He was a 27-year-old artillery captain when he was arrested on the German front in 1945 and sentenced to eight years for anti-Soviet activity. From there came a downward spiral though the prison-camps of the Soviet Union into the Gulag.

Solzhenitsyn has always been secretive and unfriendly towards snoopers, and any biographer faces a shortage of facts. Much of what we know about the man has been written by himself, in the form of novels, or by his enemies when he was on the KGB hit-list, or by his first wife, who had no cause to be kind.

Thomas, a novelist himself, has a good way of dealing with this lack of facts: he makes them up. Even before the conception of young Alexander (Sanya), Thomas is imagining the courtship of his mother and father. During the First World War, Isaaki Solzhenitsyn returns unexpectedly. His wife, Taissa, walking in her sister's garden "hears the snap of a twig just behind her, and turns to see... "

More often than not this scene-painting works. The same can't be said for Thomas's weakness for Freudian analysis. Freud looks out of place in Solzhenitsyn's brooding presence: bright and meretricious, like a dancing- master smuggled into a monastery.

But these are small nuisances. Thomas has written a riveting, passionate biography, worthy of its subject; a Russian Life, full of death, enthusiasm and love. Without this kind of heartfelt engagement, no amount of research about Russia will get you anywhere.

It is a double Life. Behind Solzhenitsyn stands his former wife, Natalya (Natasha) Reshtovskaya. Thomas uses her as a mirror to reflect Solzhenitsyn's character through a portrayal of the woman who divorced and then remarried him, and whom Solzhenitsyn also pursued and abandoned. Their lives are set against the murderous terror of Lenin and Stalin which, in turn, is set against the mute, ancient suffering of Mother Russia.

These interlocking images are very Russian - and essential: no linear biography would have been possible. Solzhenitsyn is a Promethean figure. He would not give in, he did not die, and he could not forget. His job was to keep the book of the graveyard.

Consider the dead. Taking into account revolution, famine, war, Thomas reckons the figure to be around 66 million. And that does not include those, like the poet Alexander Blok, who escaped execution but who, Russian admirers will tell you, "died of death".

Solzhenitsyn's way was to look at a single life. He might have chosen a man like the engineer Orachevsky, one of his fellow forced-labourers, who got eight years "for being seen smiling at something in Pravda". Instead, Solzhenitsyn chose a peasant. One Day In The Life Of Ivan Denisovitch, in 1963, made his name. Even Pravda approved, and Western liberals welcomed a "dissident".

They were all mistaken. Solzhenitsyn's only loyalty was to the pact he had made with the lost souls of the camps. It is a grand unforgetting. If they drowned in the prison ships of Kolyma, he will count them; if they lie in unmarked graves, he will name them; if they fuse together in unimaginable numbers, he will give life to a single man. He will do housework in hell, and turn it into a triumph.

The Gulag Archipelago is as gargantuan as the underworld it recalled. And it is founded on a surprisingly modest, devastatingly effective asset: memory. In times of terrified silence simply to remember is an act of war.

Thomas is surely right when he says that Solzhenitsyn was at his most dangerous when quite unknown. Released from prison in 1953, the authorities had another gift for him. He was banished to faraway Kazakhstan. There he became a schoolteacher. He had no name; no prospect, no publications. Instead he had "memories, and imagination!". The First Circle and The Gulag were conceived in Kazakhstan. Solzhenitsyn became a fighter who believed his books were "mines primed and ready to explode". What happened to that fighter?

Thomas draws a parallel with another preacher-writer, Tolstoy. Though the literary resemblances seem rather scant, some things chime: the ever- growing sense of mission and self-importance, the useful hypocrisy and the view that money and the liberal values of the despised West may be enjoyed, and piously dumped when convenient.

And then there is sex. Thomas recounts the long list of willing female helpers, wives, secretaries, mistresses and adoring blue-stockings who fell for Sanya's exalted sense of mission. His first wife Natasha was a casualty. Solzhenitsyn selected her friends, censored her reading, refused to have children with her, cut short her musical career, and at 50 left her for a woman half her age. The man who felt so keenly for political prisoners kept one of his own.

In 1975 Solzhenitsyn married another Natasha (or Alya). He was 50, she was 28. One of Alya's attractions, Thomas suggests, was her willingness to share his certainty that his designs coincided with all Russians'. When Andrei Sakharov called for Jews to be allowed to leave the Soviet Union, he was told by Alya that Solzhenitsyn believed the salvation of the Russian people came first. Elena Bonner, Sakharov's wife, cut through this humbug. "Don't give me that Russian people shit. You make breakfast for your own children, not for the whole Russian people."

The Politburo expelled Solzhenitsyn from the Writers Union. The Nobel Prize, in 1969, made arrest difficult, but Brezhnev was determined to strip him of his nationality. The publication of The Gulag Archipelago in Paris in 1973 was the last straw. Solzhenitsyn was imprisoned in Moscow. It was almost like old times, but he was put on a plane to West Germany and another exile, this time in America. Solzhenitsyn's words as he landed on foreign soil seem right: "I have fulfilled my duty to those who perished."

The hermit of Vermont - 1977 to 1994 - was a canny fellow, combining extraordinary secrecy and shrewd exploitation of the public media. Vermont meant lashing himself to The Red Wheel, digesting shoals of archive material and working day and night at the monumental sequence of novels which begins with August 1914. Thomas calls it a "cosmic cottage industry". And it is both magnificent and deeply sad. In Vermont, the artist died and rose again as a historian.

In 1994, Solzhenitsyn went home to Russia. His return was a royal, even a papal, progress, tinged by dark comedy. It is a terrible thing for a writer to be overcome by a sense of his own dignity. Time will tear him down, or the young will jeer - if he's lucky. Worse, they will simply fail to recognise him.

First stop was the Gulag, to commune with the ghosts he had honoured. Then a plane to Vladivostok and a train ride across country accompanied by a BBC film crew. Finally, home to Moscow where he lectured to anyone who would listen on - what else? - what was to be done. "A Slavophile government inspector," said one critic, not inaccurately. And then the man, who had so hated television in America that he rationed his children to one cartoon a week, became the host on prime-time TV. Meetings With Solzhenitsyn didn't last; it died of boredom.

Thomas subtitles his book "A Century In His Life". It's a good idea; we do see the Russian century reflected in Solzhenitsyn's life. It begins in 1918, continues with what Robert Conquest called the Great Terror, and it ends, not with a bang but a chat-show.