Moore stars as Elisabeth Sparkle, a one-time movie star who has aged into being an aerobics goddess. Now she’s aging out of that, at least according to her grotesque and piggish boss, pointedly named Harvey (Dennis Quaid), who is photographed in some spectacularly unflattering wide angles. Even before we meet Elisabeth, her career trajectory is wittily relayed in a montage that shows her Hollywood Walk of Fame star being laid, dedicated, and then subject to the ravages of time until someone finally splatters it with ketchup.

After a car accident, Elisabeth learns of a mysterious product called “the substance.” It unlocks DNA and divides cells. (This process, illustrated with an egg yolk, is the first thing viewers see in the movie, to the extent that it might be confused with a production company logo.) Elisabeth will be split in two: She’ll be both herself and an ostensibly better version of herself. The only catch is that she’ll have to switch to the other body every seven days, with no wiggle room on timing.

Elisabeth’s preparations for her first injection make for a genuinely great scene. The instructions from the makers of the substance are sparse (but come in helpful all-caps), and Moore’s character is forced to figure out on her own how to use the various packets and medical supplies in the kit. The transformation, as Elisabeth, nude in a white-tiled bathroom, experiences the sensation of having her eye double in its socket and her spinal column open to allow a birth, is legitimately inventive. Elisabeth’s double emerges and looks at herself in the mirror—and instead of Moore staring back, it’s Qualley. The Qualley version of the character, who starts calling herself Sue, quickly auditions to be Elisabeth’s replacement on the aerobics show.

You could criticize “The Substance” as having little that’s new. The deferred-aging premise is at least as old as “The Picture of Dorian Gray,” and the body-horror effects owe a lot to David Cronenberg, whose own new feature, “The Shrouds,” is set to premiere at Cannes tomorrow. But “The Substance” has its own way of handling psychology and metaphor. (The menstrual analogies—the calendaring of the body swaps, the ill-timed bouts of bleeding—are manifold.)

Fargeat’s previous feature, “Revenge” was mainly notable for an impressively drawn-out cat-and-mouse chase through a house near the end, but “The Substance” allows her to paint (in blood) on a much bigger canvas. And while Qualley has proved her facility with smiley malevolence before (in this fest, no less), Moore has never had the opportunity to tear into a role like this. The only logical move is for the Cannes jury to give them a shared best-actress prize and force them to mail it to each other every week.