Mitch Preston (Robert De Niro) plays a veteran Los Angeles detective with a dour personality. The opening scene, in which De Niro gives a talk to elementary school peppered with salty language, could very well be his funniest moment ever on film.

When an undercover drug bust goes sour because of interference from an L.A. television news crew, Mitch punches out a cameraman and destroys his equipment. The station executives threaten to sue the LAPD but agree to refrain from legal action if Mitch agrees to star in a “reality based” cop show. The network honchos figure the fiery Preston would attract big ratings. The police department likes the idea because it beats settling a potentially costly lawsuit and could turn out to be a public relations boon.

The show’s producer, Chase Rienzi (Rene Russo), likes Mitch but feels his gruffness would work better if he had a partner, particularly one who is his polar opposite. Enter Trey Sellars (Eddie Murphy), a cop who desperately wants to be an actor. He even takes his squad car during lunch breaks for auditions on movie studio lots. Sellars becomes Preston’s’ partner, and the TV cop show called “Showtime” is born.

“Showtime,” the film, gets a passing grade because both Robert De Niro and Eddie Murphy give it their all. Few are better at the slow-burn scowl as De Niro, while Murphy gets a chance to show off his trademark, horse-like laugh. There is surprisingly good chemistry between them, and they make this a better film than it deserves to be.

Also helping out in humorous cameo roles as themselves are attorney Johnnie Cochran and old T.J. Hooker himself, William Shatner, who tries to show the “real cops” here how TV cops fight crime. Flushing native Peter Jacobson, best known for being Fran Drescher’s ex-husband, is a hoot as a TV exec who greenlights “Showtime” for his ratings-challenged network.

Make no mistake about it, however: “Showtime” does have its minuses. Rene Russo, usually a welcomed presence in any movie, looks lost here. While attempting to satirize film and television police shows, the filmmakers actually wind up following many of their conventional rules, such as the nonsensical use of rapid-fire guns, crazed car chases, and blazing destruction of property. Even worse, the chief villain is yet another South American Armani-clad drug dealer named Vargas (Pedro Damian) who looks like he stepped off the set of “Miami Vice.”

“Showtime” is worth watching, but you leave the theater a bit frustrated knowing that it could have easily been better.