At his peak, he was a global celebrity whose wealth and brilliance attracted countless beautiful women, but in his old age he became an eccentric recluse who could barely articulate a coherent idea. That’s the narrative of Howard Hughes, though at the moment it’s tempting to apply it to Warren Beatty, who plays Hughes in “Rules Don’t Apply,” the disastrous new film he also wrote and directed.

A self-serving, half-baked calamity of a comedy, “Rules Don’t Apply” is set in the early 1960s, when Hughes was entering his late nutty period. The film tells us Hughes held his employees to a strict moral code, and therein lies a story of the forbidden love between Marla Mabrey (Lily Collins), a young actress on Hughes’ payroll, and her amiable driver, Frank Forbes (Alden Ehrenreich). Any hanky-panky, Hughes says, and “I’d have to fire ’em.”

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What’s the reason for this odd, asymmetrical triangle? First, it allows Hughes to be played mostly for laughs. This isn’t the tortured soul we saw in Martin Scorsese’s “The Aviator” but a lovable old kook who honks a saxophone mid-sentence and eats gallons of banana-nut ice cream. Second, it provides the movie with an age-appropriate romance. After all, the 79-year-old Beatty must know that his days of on-screen sex with 27-year-olds are over . . . right?

No such luck. In the film’s cringe-worthiest (and lengthiest) scene, Marla and Hughes paw each other in a fit of slobbering lust. This is the moment we turn against Marla’s supposedly noble character, and against Beatty as the movie’s creator. From here on, even the fine cast — Matthew Broderick, Alec Baldwin, Annette Bening and Ed Harris have small roles — can’t save this misguided movie.

“Rules Don’t Apply” has no idea what it’s doing. It’s a bittersweet, screwball biopic about a fictional chauffeur who wants to be a real-estate developer and a fictional actress who writes songs (Marla provides the film’s title theme). We’ll always remember Beatty for his stellar acting career, from “Bonnie and Clyde” to “Shampoo,” and for making at least one masterpiece, “Reds” (1981). But “Rules Don’t Apply” dents his legacy almost as badly as “Ishtar.”