Diner is another replay of ''American Graffiti,'' set in Baltimore during the Christmas season of 1959. Its vulgar language and preoccupation with sex reflect the lingering adolescence of its characters, who are poised on the brink of adulthood and can't quite find the courage to plunge in. Still, this is Hollywood's most mature treatment of the '50s-nostalgia theme so far, and the most accurate. It's not as subtle or sympathetic as the underrated ''More American Graffiti,'' laid in the early '60s, but then, that was a flop at the box office. By contrast, ''Diner'' looks like a hit, despite the secondhand feel of its plot and many of its situations.
The main virtue of ''Diner'' is its insistence on treating its characters as young adults rather than overgrown children. There are plenty of childish high jinks, some of which approach the ''Animal House'' level of cheap vulgarity. But other scenes intelligently depict a credible group of folks in their early 20s, coming to grips with real problems of life and love. Such moments are carefully directed by filmmaker Barry Levinson, and performed with conviction by a good cast that includes Mickey Rourke, the memorable arsonist of ''Body Heat'' and new-crowned king of the post-Fonzie school of acting.
With so much going for it, then, why doesn't ''Diner'' emerge as a major statement? Mostly because of the recurring weakness of its screenplay. Levinson is very good at concocting situations, and quite poor at figuring out what to do with them. Nearly every thread in the story line is unconvincingly resolved, petering out just when a touch of drama is needed. ''Diner'' looks good, but doesn't add up to much.