At first blush “I Feel Pretty” has a promising premise. It’s about a sad duckling who’s far from ugly, just pleasantly plump and plagued by self-doubt. Suddenly, magically, she believes that she’s drop-dead gorgeous, and her delusion releases her joyous, lovable self. At second blush, after seeing the film, it’s a botched premise. At third blush, after thinking about the film, it never had a chance of being much good in the hands of filmmakers whose take on their subject is sitcom-deep.

The duckling, Renee Bennett, is played by Amy Schumer. She knows how to sell a comic notion, and she commits to selling this one, come what may. Renee’s only wish is to be beautiful. She works for a global cosmetics company modeled on Revlon or L’Oreal, but she does so from the company’s online office, a squalid warren in a Chinatown basement. (Where her only co-worker, a guy, is a total dimwit, and a slob.) This establishes two things—the heroine’s obsession with physical appearance, and the movie’s devotion to selling plot points, however absurd they may be.

And however arbitrary. Renee’s imagined transformation follows her watching TV and happening onto a key scene of “Big”—the one in which the little kid tells a carnival machine, “I wish I were big.” This is a praiseworthy example of truth in borrowing. But it’s also a reminder to be careful what you wish for; some instant transformations have wondrous consequences, while some don’t. This one doesn’t, notwithstanding Amy Schumer’s desperate efforts, and despite our wishing the movie well for its message that beauty comes from within.

After the kid in “Big” wakes up the next morning and is big enough to be played by Tom Hanks, our delight extends beyond the physical change to the sweetly naïve 13-year- old point of view that the ostensibly mature hero brings to the adult world around him. When Renee transmogrifies miraculously the day after she sees the scene from “Big,” it’s all in her head, so the rest of the movie depends completely on her behavior, which is comically delusional if you buy it, and scarily demented if you don’t.

All the beautiful people in her company buy it, even when Renee talks febrile nonsense about being a supermodel, and no one asks any questions. The men around her buy it. They tell her she’s dazzling and perfect. But the compliments are unearned, because she keeps coming on like a runaway narcissist, the kind of woman any sensible man would flee in a New York nanosecond. Renee eventually reverts to her original self, as in “Big.” She’s duly cured of her paralyzing insecurity, and gets to deliver an earnest exhortation about the need for girls and women alike to believe in themselves. But you can’t believe in “I Feel Pretty.” It’s a put-up job about a self- enchanted Cinderella.

I’m Joe Morgenstern. I’ll be back next week with a review of “The Avengers.”