Crabwalk, by Günter Grass, trans. Krishna Winston

Neal Ascherson plunges into the murky waters of postwar German history as the country's leading writer brings a forgotten tragedy to the surface again

Fact - espeically the fact of what can happen to civilians in war – is not only stranger but more horrifying than fiction. Sheer awe at what really took place in the Second World War seems to be sobering two of the wildest imaginations from the European postwar generation. Roman Polanski has filmed a grave, accurate version of The Pianist, a memoir of the Warsaw ghetto. Now Günter Grass, the most turbidly fantastic of novelists, has turned into fiction the sinking of the Wilhelm Gustloff, the worst maritime disaster in history.

Even more than the bombing of Dresden, the fate of this ship remained taboo in both Germanies for many years, and almost unknown outside. In the final stages of the war, as the Soviet spearheads advanced along the Baltic coast, the great passenger liner set out from Gdynia crammed with German refugee families, trainee U-boat crews and wounded soldiers from the Eastern front. Nobody knows how many were on board. It was certainly more than 6,000, probably nearer 10,000. During the night of 30 January 1945, the liner was sunk by three torpedoes from a Soviet submarine. More than 1,200 survivors were rescued. The rest, almost all women and children, died in the freezing water.

Grass tells this story as a fable of modern Germany, a land unable to escape from its nightmarish past because it is still, in many ways, unable to face it. Crabwalk shocked many German readers by seeming to endorse grievances against Nazi Germany's conquerors. Even more startling, the novel suggests that the Nazi regime's "Strength Through Joy" cruises for workers (for which the liner was built) had much to be admired. But Grass, a sturdy social democrat, is merely trying to breach the silence of political correctness.

Readers of Cat and Mouse, one of his earliest novels, will remember Tulla Pokriefke, the scrawny, promiscuous teenager who worked as a tram conductor in Danzig. Now we discover that pregnant Tulla became a survivor of the Wilhelm Gustloff and gave birth to a son on a rescue vessel. Tulla, whose hair turned white overnight after seeing "all them little children, head down in the water", is the central figure. Growing up into a cantankerous caricature of a Mother Courage, she takes over her teenage grandson Konrad in the Nineties and fills his head with tales of the ship and its dreadful end.

Tulla's own views are chaotic. She joined the Communist Party in East Germany, wept when Stalin died but appalled her cell by arguing that "Strength Through Joy" was true socialism. Little Konrad, however, sets up a neo-Nazi website to revive the memory of the original Wilhelm Gustloff, a Nazi "martyr" assassinated before the war by the Jewish student David Frankfurter. Posing in cyberspace as "Wilhelm", he soon receives furious messages from an unknown "David"; anti-Semitic abuse is countered with withering abuse of Nazi brutality and hypocrisy.

Here, Grass is constructing a powerful allegory. Two basically innocent teenagers, asphyxiated by the rules of what must not be said, are trying to assert their identities by impersonating the past. Konrad feels a deluded mission to proclaim the sufferings of Germany inflicted by the "world Jewish conspiracy". He does not know that "David" is not Jewish, but a German boy obsessed by the Holocaust who wears a yarmulke and entreats his puzzled mother to cook kosher food. The two sense that their duel is drawing them together, and agree to meet where Gustloff's memorial once stood. But the meeting ends in tragedy, as "Wilhelm" pulls a gun and wreaks vengeance on "David".

This is a novel rich with interwoven narratives (although the translation is a bit rocky; it was the US Eighth Airforce, not "Eighth Airborne Division", that bombed Danzig). One strand follows the fate of Marinesko, commander of the Soviet submarine, who becomes drunk and insubordinate and lands up in a camp. A second is the tale of Paul, the narrator, Tulla's son and Konrad's father; his compromises as a hack journalist and his failures as husband and father provide a typical Grass satire on the moral weakness of middle-class German life. Paul is driven to write the story by a shadowy old writer (read: Günter Grass) who knew Tulla in Danzig. It's an arch device, not necessary to this cool and compelling work of piety to the forgotten past.

Neal Ascherson's new book is 'Stone Voices: the search for Scotland' (Granta)