Director Arnon Goldfinger and his mother, Hannah Goldfinger, make some unexpected family discoveries in “The Flat.”

It’s not until Grandma dies that a family realizes just how much junk she has in her closets, her purses and every kitchen cabinet. Usually, it’s a flood of old report cards, telephone bills, photos of people long forgotten — nothing important. That wasn’t the case for Arnon Goldfinger, director of “The Flat.”

When his grandmother died in Tel Aviv at 98, Goldfinger brought his camera as he, his mother and his cousins cleaned out her jam-packed apartment. They found a forest’s worth of letters, including some from a Baron von Mildenstein and his wife. It seems Gerda and Kurt Tuchler, Goldfinger’s grandparents, were close friends with the German couple. And Baron von Mildenstein was a high-ranking leader in the SS.

Goldfinger’s documentary takes on the inevitable questions that arise: How could a Jewish couple be friends with members of the Nazi party — not only before World War II, but after it? How can someone hate a certain group while maintaining a relationship with an individual in that group?

“In the morning, [von Mildenstein] goes to the office and meditates on, ‘Hmmm, what do we do with the Jewish people?’ But in the afternoon, he goes and has schnapps with one [Jewish] guy,” Goldfinger says. “Yes, he has ideology, but he also has friends, he has love. To say that about a Nazi, it’s, ‘Ooh, how can this be?’ But in reality, it’s obvious.”

Later, Goldfinger’s mother travels with him to Germany to visit the von Mildensteins’ daughter. The women are carefully polite, deliberately ignoring their shared past. It’s Goldfinger who asks the questions — an experience he says is common. The generation immediately after World War II, both German and Jewish, didn’t ask their parents questions for fear of bringing up painful memories. As one woman tells Goldfinger in the film, “Only the third generation asks questions.”

Goldfinger says it’s imperative that the grandchildren of those who lived through World War II talk to those who remain before they all pass away. “I felt that I must tell the story in order to release something,” he says. “The hiding of this story is also the hiding of deep emotions; to free some of it, I felt I must tell it.”

Landmark E Street Cinema, 555 11th St. NW; opens Fri.; 202-452-7672. (Metro Center)