The search for the truth in the remarkable documentary “The Flat” begins in modern-day Israel.

After the death of Grandma Gerda at age 98, her family begins the long process of cleaning out the apartment that’s like a slice of pre-war Berlin life. There are lots of gloves. Lots of bags. Lots of shoes. And lots of books. It’s hoarding with panache.

Yet it’s a newspaper clipping about a Nazi in Palestine that most intrigues the family, and especially her grandson, Arnon Gold-finger, also the film’s director.

Turns out his German grandparents, who escaped the Holocaust by immigrating to Palestine, were close friends with a high official in the S.S. and his wife before — and, more surprisingly, after — World War II. The official was associated with Nazi propaganda minister Joseph Goebbels and was the predecessor to Adolf Eichmann.

Goldfinger looks for clues about how this relationship happened, interviewing his mother, family friends, experts and the charming daughter of Leopold von Mildenstein, the Nazi in question. There are startling revelations, family bonding, guilt and lots of people in denial.

The movie feels more like a thriller and a mystery than a documentary. Perhaps someday, someone will be inspired to dramatize this astonishing story.

(At the Tivoli.)