Johannes Piehl, who commanded a paramilitary police battalion which killed hundreds of Jewish and Polish partisans in the Second World War, is attempting to escape from a life sentence by using a law designed to quash the convictions of political prisoners of communist East Germany.
No one doubts he was involved in large-scale killing. He is not claiming he is a victim of mistaken identity. Instead, Piehl alleges that his actions might not have led to a war crimes conviction if his 1981 trial had been heard in a West German court rather than Saxony in the East.
The appeal is potentially the most controversial case to emerge from the sweeping changes brought about by West Germany's abolition of the East's criminal justice system. The legal revolution has led to jails being emptied and then closed, the introduction of liberal laws and the sacking of thousands of East German lawyers, judges, police and prison officers who were tainted by association with the GDR.
After unification, all those who could show that they were political prisoners - such as East Germans jailed for trying to escape over the Berlin Wall - or that they were the victims of sentences which were excessively severe by the standards of the west, have been freed. Amnesties and the quashing of convictions reduced the prison population of the East from 31,000 in October 1989 to just 3,262 in January 1992.
Piehl, who is 79, has won the first round of his court battle to join the list of rehabilitated offenders. But the prosecution has appealed, and the old Nazi's case is now before Cornelius Prittwitz, a liberal lawyer brought in from the west as professor of law at Rostock University in east Germany, and an appeal judge on the bench of the supreme regional court.
Professor Prittwitz, who is working to bring western law to people who have known only authoritarian regimes since 1933, said he was disturbed by the issues involved.
No details of Piehl's attempt to get out of prison have appeared in Germany. Nor, if he is freed, is there likely to be any public announcement. The judge accepted that Piehl's attempt to be rehabilitated was a ''private justice'' procedure.
''I do not want headlines saying western judge frees Nazi,'' he said. ''That would be damaging. The questions are far more subtle.''
The fact that Professor Prittwitz will make his decision in Rostock - scene of attacks by neo-Nazi hooligans on a block of flats housing immigrants in 1992 - makes the case even more sensitive.
Piehl was first the second-in-command and then the commander of a Nazi police battalion in the Polish region of Katowice. There were battles with partisans in 12 villages, in which large numbers of resistance fighters were killed. The court papers show that in one village about 100 men, almost all Jews, were executed.
After the war Piehl went to live in communist East Germany and, said Professor Prittwitz drily, became a ''good and useful socialist citizen''. He found a job as a senior sports administrator in Saxony and was left undisturbed until the Poles came to the East Germans with evidence about the killings.
When the Wall came down in 1989, he appealed to be included in a wide-ranging amnesty for prisoners. But his pleas were rejected. Now, after invoking the rehabilitation laws, only Professor Prittwitz stands between him and freedom.
For the communist court which sentenced him in 1981 the case was simple. The invasions of Poland and the Soviet Union were illegal. Anyone involved in massacres was by definition a war criminal.
In the old West Germany, Professor Prittwitz said, the case would not have been so clear-cut. To kill partisans would not necessarily be a war crime: it would only become one if it could be proved that they were executed because of their race. Piehl appears to be claiming that Jews were likely to be partisans and were executed because they were fighting, not because of their religion. The huge purge of former communist court, police and prison officials, which followed the introduction of western law - as well as the wholesale sacking of law lecturers - means there are many ready to pounce on any mistake in the judgment.
The purge, and the apparent failure of the west to raise east Germans' living standards quickly, has brought bitterness. In last month's German elections, the reformed Communist Party won 40 per cent of the vote in the former East Berlin .
Professor Prittwitz's smile fades when he thinks about the decision he has to make and its consequences. The question he asks is whether everything the old East Germany did was necessarily wrong.
''This was not a straightforward political conviction,'' he said. ''It was not outside the framework of what was normal law in the east. I have to ask, do we really want to overturn every decision that the East Germans made?''
The judge will reach his verdict in three months. He will not make it public. Piehl can announce his rehabilitation if he is freed. But it is unlikely that he will want to draw attention to himself.