A fifth of the art treasures of Europe were plundered by the Third Reich. Adolf Hitler was quite simply the greatest art thief of all time.

Friedrich and Louise Gutman were two victims of this institutionalised robbery. Leading bankers in Holland before the War, they declined to hand over their art collection to the occupying Nazis. In spite of assurances about their safety from Himmler, they were sent to Theresienstadt concentration camp, where they were savagely interrogated. When they still would not give the Germans their art-works, Friedrich was beaten to death, Louise was transported to Auschwitz where she was gassed, and the Nazis seized the collection.

The consequences of this crime are still reverberating more than 50 years later. As "Making a Killing", a compelling edition of Arthouse on Channel 4 tomorrow night, shows, the Gutman family have spent the last half a century trying to recover the works of art. Thirteen of their paintings are still missing.

Attention is centred on a Degas painting, Landscape with Smokestacks, which was once owned by Friedrich Gutman and has finally been tracked down to the collection of Daniel Searle, a pharmaceuticals billionaire from Chicago who bought it in good faith 10 years ago. Next month, the Gutman family are going to court in the US in an attempt to retrieve the picture. The practices of the art world will also be on trial.

The family have been helped by a Washington detective, Willi Korte, who specialises in finding stolen art and is known as "the Indiana Jones of the art world". He has his own concerns: "Nobody who is part of that community has any interest in helping victims to recover their paintings. The missing works of art have never really been `lost'. It's just that the art world preferred not to ask too many questions. These works were given sanitised histories. Everyone turned a blind eye and made a killing."

Nick Goodman, Friedrich and Louise's grandson, does not have a very high opinion of the art world, either. "People are making a lot of money, and they're selling paintings without checking provenance carefully enough - either willfully or just by blind ignorance - and when inquiries are made, you're stonewalled."

Making the documentary has certainly not filled producer Anne Webber with respect for the art market. "It does not come well out of this film. It's the largest unregulated business in the world. It's a multi-million- pound business and needs some regulation. Victims of stolen art need some protection. Museums should take responsibility for what they buy. It was interesting to discover that provenance is such a fallible instrument. A car at least has a registration document which is a legal document. People think that assurances about provenance have the same weight, but they don't. Some collectors have discovered that at great cost.

"For instance, one has to look particularly carefully at any Impressionist painting that appeared after 1939 because the Nazis stole a huge number of them and traded them on."

The legal action has taken its toll on the Gutmans. Lili, the now-elderly daughter of Friedrich and Louise, is finding the process especially stressful. In some ways she regrets ever starting the action. "I don't even want to imagine things because it's too painful for me," she says. "Still now after all these years, it makes me feel sick."

According to Webber, "there is no financial or emotional advantage for the Gutmans in pursuing this, other than trying to resolve damage that happened in the past. It's been very sad for Lili. The reawakening of the past has been very painful for her. At pre-trial hearings, she was cross-examined over three days and asked fantastically detailed questions about her life. For a woman of 77, it's hard to bring up things that are so painful. She had heart trouble last year as a result of all this."

Matters are not made any easier by the fact that Lili feels bitter because her version of events is being contested in court. "They're trying to reconstruct the past out of pieces of paper and not out of what one remembers in one's mind and in one's heart, which is much more important," she says. "Still, lawyers, they want papers."

The ultimate irony is that even if the Gutmans triumph in the court case, they won't be able to keep the Degas. They'll have to sell it to pay the only winners in this whole sorry affair: the lawyers.

`Arthouse: Making a Killing' is on tomorrow at 7pm on C4