After an exceptionally promising triple-threat debut as the director, co-writer and co-star of "Real Life," the freshest American movie comedy of 1978, Albert Brooks has returned with a discouraging thud.

His second feature, "Modern Romance," opening today at area theaters, stirs only faint ripples of amusement. Attempting a Los Angeles variation on the sort of misalliance made popular by Woody Allen's "Annie Hall" -- a romantic attachment between temperamental and ethnic opposites undermined by male vanity and possessiveness -- Brooks leaves himself in a number of inescapable binds.

The movie begins as Brooks, cast as a melancholy Hollywood film editor named Robert Cole, calls it quits with his beautiful, long-suffering fiance, Kathryn Harrold as a bank officer named Mary Harvard. "Okay," she sighs, "it's over again." Miffed, he insists "Not again. This is it. You've heard of a no-win situation? Vietnam. This. . ."

The date breaks up on a note of mutual hostility. Mary goes her own way, leaving us to ponder, in considerable but rarely funny or appealing intimacy, the despondency of Robert. Too upset to work, he entrusts the night's editing chore (tightening a sequence in a science-fiction cheapie starring George Kennedy, who later turns up at a party playing himself) to his assistant, Jay, agreeably played by Bruno Kirby. Robert also does himself with a couple of Quaaludes, and for the next reel or so we watch Brooks try to sustain a slapstick monologue in which Robert returns home, makes and receives a series of phone calls and eventually passes out to culminate a solitary evening of brooding and muttering about his shattered love affair.

It's an audacious but extremely periolous solo performing stunt. Brooks seems to rescue it from numbed disaster in midpassage, hitting on a witty facsimile of slurry disorientation and self-pity after languishing without a spontaneous laugh for what feels like hours.

Although the sequence rebounds, the movie never overcomes the initial tendency to sputter and stall. The plot goes on to confirm Robert as an incorrigibly smitten, insecure suitor. He can't stay away from Mary, but as soon as he arranges a reconciliation, he backslides and goes on another jealous binge of snooping or nudging that threatens to provoke another separation. In short, he's impossible: a stoogy amalgam of Othello and Iago who insists on provoking himself into moods of jealous panic or embarrassment.

A character as insufferable as Robert seems a poor bet to carry as popular romantic comedy. Brooks loses the gamble decisively because this role seems too morose and introverted for him. What's worse, he allows this inferior role to become almost a oneman show. It's the reverse of the situation in "Real Life," where he played a comic-turned-filmmaker who couldn't help hogging the limelight and distorting the raw material when he embarked on a grandiose documentary project -- recording a year in the life of a typical American family.

In "Real Life," the Brooks character was enhanced by the presence of strong foils in Charles Grodin, Frances Lee McCain and others. But in "Modern Romance" you begin getting nervous at Brooks' very isolation. During the druggy monologue his only exchanges are with disembodied telephone voices and a pet bird, and these interlocutors don't help him fill out a humorous characterization. The story also suffers by failing to account for a mutual romantic folly. Mary never quite gets into the picture, yet she's evidently willing to tolerate and forgive Robert indefinitely. If he has a lovable side, Brooks neglects to turn it screenside up. Mary's strange refusal to get fed up with Robert's behavior surely makes her a suitable candidate for ridicule, but the satiric viewpoint remains one-sided.

The brainy, abrasive aspects of Brooks' personality are probably best protected by being flaunted satirically rather than toned down. One can imagine him filling a modern rogue's gallery of dubious hard-chargers and overachievers -- a show-biz personality pursuing political office, the social director on a truly uninhibited love boat, a TV programming executive blithely engineering the Silliest Season Ever. His talent for forcefully obnoxious manipulators may be wasted on small-time jerks like Robert.

The most relieable sources of humor are inside jokes about the contemporary film business. There's a nifty fleeting reference to "Heaven's Gate" and an only-in-Hollywood kind of ice-breaker, "We met at the 'Nickelodeon' wrap party, right?" But even at this level the payoffs are scantier than one might hope.The incidental jokes aren't nearly as plentiful as they were in "Real Life," and Brooks fails to exploit a potentially funny connection between Robert's romantic and professional vicissitudes.

It's not easy for a comedian to establish or sustain an identity with a movie audience. Considering his originality and promise, Brooks shouldn't be begrudged the occasional wrong number like "Modern Romance." Still, he's in a spot: He may be obliged to deliver commercially or else next time around if "Modern Romance" makes as little impression on the box office as "Real Life," which deserved to rally a public. "Modern Romance" is likely to interest only those who wish Brooks well and don't mind making extravagant allowances.