THE POSTER TAGLINE was “You’ll believe a man can fly,” but before Dec. 15, 1978, it was some feat even to imagine how an earnest movie about a man in a red cape and tights could ever get off the ground.

So much was so different on the space-pop continuum then. It was a Hollywoodland before entire CGI cities and cinematic universes and online fandoms — but, especially, before the discovered charm of Christopher Reeve.

Today, six of the nine top-grossing movies of the year are superhero stories — and this month brings “Spider-Man: Into the Spider-Verse” (out this weekend) and “Aquaman” (Dec. 21), plus the PG-13 sequel recut “Once Upon a Deadpool.” And when trying to pinpoint just how Hollywood got here — with most of its billions in box-office receipts swaddled in spandex — the releases of “Batman” (1989) and “X-Men” (2000), “Spider-Man” (2002) and “Iron Man” (2008) stand like crucial mile markers.

Yet director Richard Donner’s “Superman: The Movie,” which celebrates the 40th anniversary of its opening Saturday, is the true father of superhero cinema — the first such feature project that captured the masses while also refusing to steer hard toward camp (as had “Superman and the Mole-Men” in the ’50s and the TV series-related “Batman: The Movie” feature in the ’60s).

In the years just before the debut of “Superman” — which has just received a 4K restoration release to mark the anniversary — 1975’s “Jaws” birthed the summer-season blockbuster, and 1977’s “Star Wars” (partly inspired by Flash Gordon comics) ushered in a new type of mass-merchandise campaign. So by the winter of 1978 — the same year “Star Wars” and “Close Encounters of the Third Kind” won Oscars for their technical achievements — audiences were primed for an epic science-fiction film adapted from a comic book.

Superman, created four decades earlier by Midwestern teenagers Jerry Siegel and Joe Shuster, was a natural candidate — an icon of Americana who also happened to be a superpowered space alien.

Warner Bros., though, wasn’t going to blaze any trails without adhering to some of the hidebound rules of making a big studio picture. That meant securing big stars, a hot director and respected screenwriters. In the wake of the huge success of the first two “Godfather” films, the “Superman” team went after star Marlon Brando and writer Mario Puzo, and Donner was hired fresh off his horror hit “The Omen” (1976).

That’s when things began to get bloated. Brando would command a $3.7 million payday and a high percentage of the back end for playing Jor-El, Superman’s father. And Puzo’s screenplay ran hundreds of pages — about five times the size of a script they could shoot. The production budget would climb to $55 million — one of the most expensive movies ever at the time.

Another A-lister, Gene Hackman, was ultimately landed to play villain Lex Luthor. But who could credibly fill the red, blue and yellow tights?

The names of seemingly every big leading man — Robert Redford, Clint Eastwood, Al Pacino, Paul Newman — were mentioned for the title role. But the great casting director Lynn Stalmaster urged consideration of Reeve, a young, lanky, 6-foot-4 Juilliard alumnus who rippled with easy charisma. Reeve also had a ready chemistry with Margot Kidder, who reportedly beat out Stockard Channing for the role of a spunky Lois Lane. (Kidder died in May.)

As mild-mannered reporter Clark Kent, Reeve could do light screwball comedy behind oversized specs — especially after studying Cary Grant in “Bringing Up Baby.” Still, the filmmakers worried: Wasn’t the 170-pound Reeve too thin for the part? Refusing a padded costume, the actor packed on the chiseled pounds with the help of David Prowse, the bodybuilder beneath the Darth Vader suit.

Reeve was hired for $250,000 — or just about what Brando ultimately made per minute in his limited screen time.

Five writers would lay hands to the screenplay, including Oscar-winning Robert Benton and legendary script doctor Tom Mankiewicz. John Williams would provide the triumphant Oscar-nominated score. And in a pre-CGI era, the practical-effects and blue-screen wizardry would boost “Superman” to a special achievement Academy Award.

The over-schedule production even weathered the drama created by the filmmakers attempting to shoot both of the first two Superman movies simultaneously. Father-and-son producers Alexander and Ilya Salkind would fire Donner before he could complete the sequel. Brando and Puzo would later sue the producers over pay.

But as opening day neared, “Superman” was ramping up as an event — even if it all seemed surreal to Reeve.

“I hardly think of myself as Superman,” Reeve told The Washington Post several weeks ahead of the debut, adding: “I think of Supie as a fantasy of ourselves; Clark is a tongue-in-cheek impression of who we are.”

Several years earlier, Reeve had been to Washington’s National Theater during tryouts ahead of his Broadway turn in “A Matter of Gravity.” A couple of return trips to the District spotlighted his gravity-defying career trajectory: He and Sen. Ted Kennedy announced the “presidential premiere” of “Superman” — a $1,000-a-seat Special Olympics benefit at the Kennedy Center on Dec. 10, 1978, that was attended by President Jimmy Carter, as well as a black-tie party at the Rockville home of Eunice Kennedy Shriver and Sargent Shriver, among other kickoff festivities in the area.

“If you have to spend two years of your life making a movie like ‘Superman’ just to meet the Kennedys, then it’s worth it,” Donner told The Post at the Shriver event (which Maria Shriver and Arnold Schwarzenegger, who had lobbied for Reeve’s role, also attended).

Five days after the Kennedy Center event, “Superman” opened nationwide in more than 700 theaters — to mostly positive reviews.

“ ‘Superman’ is a pure delight, a wondrous combination of all the old-fashioned things we never really get tired of: adventure and romance, heroes and villains, earthshaking special effects, and — you know what else? Wit,” critic Roger Ebert wrote. “That surprised me more than anything: That this big-budget epic, which was half a decade making its way to the screen, would turn out to have an intelligent sense of humor about itself.”

Post critic Gary Arnold wrote: “Despite a lull here and a lapse there, this superproduction turns out to be prodigiously inventive and enjoyable, doubly blessed by sophisticated illusionists behind the cameras and a brilliant new stellar personality in front of the cameras — Christopher Reeve, a young actor at once handsome and astute enough to rationalize the preposterous fancy of a comic-book superhero in the flesh.”

Reeve had told The Post: “This character Superman is a real part of our American myth.” And he noted of the film’s sense of spectacle and heroism: “We take the audience to a place they’d like to escape to.”

Filmgoers embraced its soaring sense of escape and reassurance: “Superman” became one of the year’s biggest films, grossing more than $300 million worldwide.

"Donner famously tacked the word ‘verisimilitude’ up around the film’s production offices to remind the cast and crew of what he felt was most important,” says Glen Weldon, NPR pop-culture host and author of “Superman: The Unauthorized Biography.” “He wanted to ground the fantasy in the real world — to better contrast who Superman was, and what he represented, to the era of the anxious 1970s.”

Forty years later, “Superman,” for all its rough edges, has only burnished its stature as a forerunner — a canary in Hollywood’s gold mine of superheroes. And he directly inspired the next generation of superhero talent, on the page and on the screen.

"Superman” was “the film that made me run through the streets afterwards with my arms raised in front of me, gimbaling left and right in between parked cars,” says Jim Lee, publisher and chief creative officer for DC Comics, who was 14 at the time. “What can I say? It had me completely enthralled and made me understand the true power of a great story is in its ability to endlessly inspire.”

And Donner’s “Superman” is “what made me fall in love with Superman and the genre as a whole,” says writer-producer Geoff Johns (“Aquaman,” the upcoming “Wonder Woman 1984”), who would work as Donner’s assistant, “and what inspired so many others to make men and women fly across the silver screen.”

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