British filmmaker Stuart Urban’s deeply affecting documentary, which seeks to uncover the truth about his father Garri’s past, often seems too incredible to be real.

Urban Senior, born in a Polish shtetl in 1916, qualified as a doctor. While attempting to swim an icy river from Soviet territory to Romania after the 1939 Nazi invasion, he was shot by Russian snipers and sent to a gulag in the Arctic Circle. He succeeded in escaping in 1940 and, amazingly, became head of the health service in Ukraine, enjoyed a romance with Moscow fashion-magazine editor Noka Kapranpova, was rearrested, imprisoned and tortured by the KGB before absconding yet again in 1946, posing as a German prisoner of war and escaping to the West.

In 1992, after the fall of the Iron Curtain, father and son travelled to search out and record significant aspects of his past. It was a quest that brilliantly illuminated Garri’s character, most notably in his reunion with his former lover Noka after 50 years. The journey, which forms the basis of this superb documentary, brought father and son closer together (“I’m doing this for you, son,” says Garri during an emotionally bruising discussion) while still leaving key personal questions unanswered. After Garri’s death in 2004, Stuart returned to the project, interviewing Garri’s former lover, Noka. The final film, integrating home movies, footage of sorties into post-Communist Russia and new interviews, makes unmissable viewing.