Hanni Weissenberg sat alone, stone-still, as the Gestapo pounded on the door.

On that cold morning in 1943, the German police had all but emptied the building of Jews while 18-year-old Hanni was at the doctor’s with a badly injured finger. By the time she returned, the family she’d been living with — friends of her late parents, who had been sickly — were gone.

Now the Gestapo was back. Hanni held her breath, gripped the edge of the table and tried to block out the noise.

“Open up, open up,” they persisted. But Hanni — “a rather shy, obedient young girl,” she tells The Post with the help of a German translator — refused. “It was very difficult not to follow that order,” she remembers. “One has to really hold on, in order not to be obedient when such a strong demand comes.”

Eventually, the officers left. Hanni realized there was only one way to avoid getting captured and deported: She had to run.

But she didn’t go far. In fact, she never left Berlin. As shown in “The Invisibles,” in theaters Friday, she and 1,500 other German Jews survived the Nazis’ annihilation without leaving Berlin.

Hanni — now Hanni Levy, a 94-year-old great-grandmother living in Paris — is one of four former Berliners who share their extraordinary stories in this 2017 German film, which cuts between traditional documentary-style interviews and dramatized scenes of their lives.

After the Gestapo stopped their door-pounding and moved on, Levy cowered in the building’s stairwell. Then — with nothing but her purse in hand and with the mandatory yellow Star of David sewn to her coat — she ran to the nearest subway stop, and showed up quavering at the doorstep of some non-Jewish friends.

It was the lady of the house, who’d known Levy’s mother, who gave her the simple idea that ultimately saved her life.

“She made me an appointment with a hairdresser,” says Levy. After a labor-intensive, three-day process, the once dark-haired Levy looked in the mirror and saw herself as a stylish, corn silk blonde.

To disguise her identity even further, she renamed herself Hannelore Winkler. “I said to myself, ‘I really have to change now,’” Levy says, during a visit this week to New York. “I’m a different person. I’m not a little girl anymore.”

Part of that transformation, she says, was deliberately letting go of her fear.

“Fear is a luxury I could not afford,” the auburn-haired Levy says today, her eyes vivid behind her glasses. “Of course, I was worried that someone would still recognize me. But if you are scared, you become insecure and that is really more dangerous than anything.”

Over the next year, Levy stayed with a few families in the neighborhood, helping out with their children and doing whatever odd jobs she could handle with her worsening finger (“I wouldn’t go to a doctor because they would take down your personal data,” she says, gesturing with the missing tip of her pointer on her right hand.) She let people believe that she, like many German civilians at the time, had lost her home in a bombing.

When she wasn’t working, she loved going to the movies, “because it was dark, it felt protected.” There, she met a handsome young man named Oscar Kolzer, who’d just been drafted into the army. Before he left, he asked Levy for a favor: Would she please keep visiting his mom, who sold tickets at the theater?

Levy said she would, and their relationship eventually became much like mother-and-daughter. Although Kolzer’s mother knew the truth, she took Levy into her home through the end of the war. “She said, ‘You can stay with me, and then perhaps somebody will help my son, too, when he is in danger.’”

After the Allies took Berlin, she and Levy, then 22, moved to Paris together.

“It took me a long time to feel safe,” says Levy, admitting she found it challenging to adjust to her new home. She didn’t speak French at first, and German was treated as the hostile language of the occupiers. She was also surprisingly naive for her age, she says: “I hadn’t lived. I had only survived.”

But in time, she moved on, married a Frenchman, had two children and returned to Berlin — “home” — whenever she could.

Levy says she’s happy that “The Invisibles” pays homage to those whom she calls the “quiet heroes”: the Germans who risked their lives to help out desperate Jews with food, money, jobs and shelter.

“It was very difficult,” she says of her remarkable journey. “But the will to live is stronger than anything else.”

“The Invisibles” opens Friday at Quad Cinema and Landmark 57 West.