A long-lost original cut of the 1927 silent film sat for 80 years in a private collection and then in the Museum of Cinema in Buenos Aires, where it was discovered in April with scratched images that hadn't been seen before.
Museum director Paula Felix-Didier said theirs is the only copy of German director Fritz Lang's complete film.
"This is the version Fritz Lang intended," said Martin Koerber, a curator at the Deutsche Kinemathek film museum in Berlin, Germany.
"Metropolis," written by Lang and his actress wife Thea von Harbou, depicts a 21st century world divided between a class of underworld workers and the "thinkers" above who control them.
Soon after its initial release at the height of Germany's Weimar Republic, distributors cut Lang's three-and-a-half-hour masterpiece into the shorter version since viewed by millions worldwide.
But a private collector carried an original version to Argentina in 1928, where it has stayed, Felix-Didier said.
In the 1980s, Argentine film fanatic Fernando Pena heard about a man who had propped up a broken projector for "hours" to screen "Metropolis" in the 1960s. But the version of the film he knew was only one-and-a-half hours long. For years, he begged Buenos Aires' museum to check their archives for the man's longer version.
This year, museum researchers finally agreed and in April uncovered the reels in the museum's archive.
In June, Felix-Didier flew with a DVD to the Friedrich Wilhelm Murnau Foundation in Wiesbaden, Germany, which owns the rights to "Metropolis." Researchers there confirmed that the scenes were original.
News of the find excited film enthusiasts worldwide.
"This is a movie that millions and millions of people have seen since its release and yet, in many ways, we've never seen the true film," said Mike Mashon, head of the Moving Image section of the U.S. Library of Congress in Washington.
"Metropolis" was reissued in the U.S. in 2002 by Kino International Corp., which owns the rights to distribute the film domestically, Kino's general manager Gary Palmucci said.
Kino may rerelease the new, complete version of the film.
Meanwhile, Buenos Aires' Museum of Cinema is holding its treasure tight.
"The film hasn't left the museum and it won't leave until the city government and the Murnau Foundation decide what to do," Felix-Didier said.