[Source: AP]

“The Purge” for sex is the best way to describe the premise of “One Night Only” unless you prefer Shirley Jackson’s “The Lottery” for one-night stands.

As the movie announces right away: The U.S. government has passed a mandate that premarital sex is illegal except for one night a year. For the past three years, sex out of wedlock has been off limits except for an annual 12-hour reprieve. “It’s been a lot,” reads the opening title cards.

Such a concept, naturally, spawns a lot of questions. How could such a policy be policed? Where, exactly, are the lines drawn? And why hasn’t everyone moved to Canada?

But “One Night Only,” directed by Will Gluck (“Easy A,” “Anyone But You”), is less interested in extrapolating the potentially very bizarre ramifications of its dystopian concept than it is selling a surprisingly tame romantic tale. “One Night Only” may be dressed up as a raunchy sex comedy, but it’s a rom-com at heart, and a chaste one at that.

That makes “One Night Only” a disappointingly denuded comedy without the gumption to interrogate its satirical gimmick. And yet its charming leads, Monica Barbaro and Callum Turner, do enough to keep “One Night Only” pleasantly watchable.

The health of the rom-com is perpetually debated, but one of the genre’s most abiding benefits is in giving young stars an uncluttered big-screen stage to shine on. Turner, the Bond-rumored British actor, has already appeared in one (also monogamy-friendly) rom-com in last year’s “Eternity.”

But “One Night Only” is an especially welcome showcase for Barbaro, who has thus far stood out largely in male-dominated ensembles ( “Top Gun: Maverick,” “A Complete Unknown,” “Crime 101” ). So, while “One Night Only” is unlikely to be a standout in her filmography, it’s the one that most proves her lead-actor incandescence.

One of the biggest stretches in a movie full of them is that these two — neither of whom are exactly hard on the eyes — are among the only ones striking out on a night dedicated to scoring.

Owen (Turner), who runs a Manhattan pizzeria, has his engagement plans thwarted when his longtime girlfriend (Maya Hawke) says she plans to sleep with someone else. Allie (Barbaro), meanwhile, is a single singer (she gets paid to belt out pharmaceutical jingles) who has to be coaxed by her friends into even going out.

Both are romantics on a night for flings. After bumping into each other and meeting on the street, they spend the night traversing New York, separately but often together, through nightclub escapades and condom shortages.