The US judicial system is a mighty juggernaut, and rarely given to second thoughts. But it now has a chance to show clemency to one of the world's most celebrated film directors.

The opportunity has arisen with the request last week by Roman Polanski, the Oscar-winning director of such movie classics as Rosemary's Baby, Chinatown and The Pianist, for his 1978 guilty plea to the charge of having unlawful sex with a 13-year-old girl to be struck down on the grounds of misconduct by both judge and prosecutor in the case.

The plea-bargain deal was reached after Polanski initially faced charges of rape, drug use and sodomy that could have brought life in jail. But even the lesser charge of unlawful sex with a minor carried a prison term. To avoid such punishment, the Polish-born director fled America to France, of which he is now a citizen, and which has made clear it will never extradite him to the US.

At the time the case was a colossal scandal. Polanski was widely seen in the US as a strange and vaguely sinister figure, despite the vast acclaim for Chinatown – still regarded as his career masterpiece – four years before. For an America far more straitlaced than it is now, he seemed symbolic of an era of licence and collapsing moral standards.

Many of his films had grotesque, paranoid or sexual themes. He was also a foreigner, with a tragic background.

He was born to Polish Jewish parents, and his mother died at Auschwitz. He might have suffered a similar fate, had he not escaped from the Krakow ghetto. In 1969 his wife, the actress Sharon Tate, pregnant with Polanski's child, was murdered by the Charles Manson cult.

Eight years later came the episode that made him a fugitive from US justice. Polanski wanted to take pictures of 13-year-old Samantha Gailey for French Vogue, which he had been asked to guest edit. Her parents gave consent.

According to the charges, at a second session in Los Angeles in 1977 Polanski plied the girl with champagne and sedatives before having sex with her. In his autobiography, Roman by Polanski, the director claimed the girl had been set up by her mother in an attempt to blackmail him.

Now attorneys for Polanski, who is 75, argue David Wells, the prosecutor in the case, improperly coached the judge, Laurence Rittenband, during the case and all the charges should be thrown out. The Los Angeles district attorney's office has thus far refused to comment.

Whether the US legal system is ready to let bygones be bygones remains to be seen.

But he has already won redemption. Earlier this year, Gailey, now Samantha Gei­mer and a 44-year-old mother of three, made her peace with him.

"I think he's sorry. I think he knows it was wrong," she said. "I don't think he's a danger to society. I don't think he needs to be locked up forever and no one besides me has ever come out and accused him of anything. It was 30 years ago now. It's an unpleasant memory... but I can live with it."