Rome wasn’t built in a day but Francis Ford Coppola’s Roman epic “Megalopolis” falls apart frequently over 138 minutes.

While the ambitions, visual style and stellar cast are there for this thing to work on paper, the sci-fi epic (★½ out of four; rated R; in theaters Friday) ultimately proves to be a disappointing, nonsensical mess of messages and metaphors from a filmmaking master. Coppola’s legend is undoubtedly secure: “Apocalypse Now” is the best war movie ever, and “The Godfather” films speak for themselves. But he's also had some serious misses (“Jack” and “Twixt,” anyone?) and this runaway chariot of incoherence definitely falls in that bucket.

The setting of this so-called “fable” is New Rome, which might as well be New York City but with a more golden, over-the-top touch. (The Statue of Liberty and Times Square get minor tweaks, and Madison Square Garden is pretty much an indoor Colosseum.) Cesar Catilina (Adam Driver) is a progressive-minded architect who heads up the city’s Design Authority and can stop time, and he plans on using this magical new building material called Megalon to soup up his decaying city.

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He’s made a lot of enemies, though, including New Rome’s corrupt and conservative major Franklyn Cicero (Giancarlo Esposito). Cicero calls Cesar a “reckless dreamer,” aiming to maintain New Rome’s status quo no matter what. However, his ire increases when his more idealistic daughter Julia (Nathalie Emmanuel) goes to work for Cesar and then becomes his love interest.

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There’s a lot of Shakespeare here, not only that “Romeo and Juliet”-ish angle but Cesar cops a whole chunk from “Macbeth” for one of his speeches trying to get the people of New Rome on board with his grand plans. Coppola’s influences are not subtle – “Metropolis,” for one, plus ancient history – and the oddball names are straight out of the pages of “Harry Potter” and “The Hunger Games” with a Times New Roman flair. Aubrey Plaza’s TV host Wow Platinum, Cesar’s on-again, off-again gal pal, sounds like she taught a semester of entertainment journalism at Hogwarts.

The supporting characters – and their actors – seem to exist just to make “Megalopolis” more bizarre than it already is. Jon Voight’s Hamilton Crassus III is a wealthy power player and Cesar’s uncle, and his son Clodio Pulcher (Shia LaBeouf) envies his cousin’s relationship with Wow and has his own political aspirations. “America’s Got Talent” ukelele wunderkind Grace VanderWaal randomly shows up as virginal pop star Vesta Sweetwater – New Rome’s own Taylor Swift of sorts. Dustin Hoffman is Cicero’s right-hand man Nush Berman, and Laurence Fishburne has the dual roles of Cesar’s driver Fundi Romaine and the narrator walking the audience through the sluggish storytelling.

Thank goodness for Esposito, who might be the antagonist but winds up grounding the film in a needed way the more it veers all over the place. (Though Plaza is deliciously outrageous.) “Megalopolis” screams to be a campy B-movie, though it’s too serious to be silly and too silly to be serious. And sure, it takes some big swings – like the use of triptychs as a storytelling device and the sight of gigantic statues just walking around town – but it’s all for naught because the story is so incoherent.

The film has been Coppola’s passion project for more than 40 years, and the result is something only his most ardent and completionist fans might appreciate.