What terrible thing would your father have to do to make you admit he was a monster? For most people, discovering he was responsible for the murder of 75,000 innocent men, women and children would be enough, but not Horst von Wächter. Horst is the gentle and disillusioned son of a high-ranking Nazi officer who ran the district of Galicia in the Ukraine during German occupation.

It was under Otto von Wächter's command that 3,500 Jews were executed in a wood in Lviv, but regardless of the facts which the Nazis documented so well, Horst will not accept it. It is this refusal to acknowledge his father's guilt that drives the narrative in David Evans's finely crafted and totally absorbing documentary, which is built around an understanding between human-rights lawyer Philippe Sands and Niklas Frank, the son of Nazi general Hans Frank who was executed at Nuremberg.

Niklas Frank is a fascinating and, dare I say, likeable man as he bitterly condemns the actions of his father, with whom he had no relationship and shares vivid recollections of visits to the Crakow ghetto, where he was old enough to realise the people looked sad.

"I poked my tongue out at a small boy through the window of the Mercedes," says Niklas and the image chills your heart. It was through Niklas that Sands met Horst, who was willing to castigate the Final Solution and all the atrocities but will not attribute blame to his father. The absence of a signature on official documents is what Horst clings to for justification but, for Sands, this is justifying the killing of his grandfather's relatives. David Evans gives all three men space to have their say as they travel to the Ukraine together and thoughtfully uses archive film that tells the stories of these sons who grew up against a backdrop of Nazi uniforms, but now hold different views. That in itself is terrifying and will doubtless be a trigger for serious debate.