Given enough microphone cord, and he always is, ABC's tie-dyed muckraker Geraldo Rivera usually tends to hang himself. Tonight, ABC hands him the full hour of its tabloid news magazine "20/20" to practice his designer-jean journalism: a blitz of visual kitsch ostensibly exposing a nationwide heroin epidemic.

Much of the footage within the hour -- at 10 tonight on Channel 7 -- is indeed strong stuff, and if Rivera again insists on tap dancing atop his soap box, at least the camera supports alarmism. Yet one may not feel as concerned and frightened as one should when the hour is over, because the report is staged as another chapter of Geraldo Rivera, Man with a Mission.

Out in Hollywood they love the term "reality-based" right now. Rivera seems to be perfecting something relatively new: reality-based news. With the cooperation of authorities (as ABC News is quick to point out), Rivera, a crew and producer Craig Rivera, Geraldo's younger brother, scour the opium dens of Pakistan, where heroin is born, and the drug-stained streets of New York's Lower East Side, and an affluent suburban home, where teen-age son Jeffrey turns out to be a junkie. Never is there a moment's doubt as to who the star is.

The hour is punctuated with hidden camera footage, Abscammy videotapes, shadowy interviews, and the kind of swashbuckling exploits that make Dan Rather's trip to Afghanistan -- or, for that matter, his cab ride around Chicago -- look like a jaunt to Palm Springs. In Pakistan, Rivera attempts to buy 2,500 kilos of opium and is led to a stockade that houses not only the drug but dozens of stolen Sony TV sets.

In Rome, he and the crew happen upon a junkie shooting up under a bridge -- and Rivera seems surprised that the kid resents the intrusion. Best of all, in New York, Rivera goes "undercover" wearing a burly beard and a bandanna over his head, to buy some street dope. A hidden camera in a truck and another in a briefcase follow him as he is led to the dealer and, in narration, he tells us how very dangerous this was.

Then, surprisingly, he stops on the street in his junkie drag and talks to the camera -- presumably safe from peril by this time. A moment later we find him at a detoxification center for one of those sermonette-lectures that only Geraldo can give: "Looking around this ward," he instructs, "two things should become perfectly clear to you. First, that this place is cramped to capacity. And secondly that there's an unmistakable, undeniable connection between heroin addiction and crime."

Often, the program aims to shock, not always productively. It opens with montages of needles going into arms and of arms badly scarred, as if the worst ravages of heroin were cosmetic. More unsettling are such scenes as a crowd of Wall Street types casually buying heroin on their lunch hour, or the description, by a well-off California mother, of how her 10-year-old daughter became so used to Mommy shooting up that she'd ask, "Did Daddy give you your hit yet?"

That the drug takes a horrible toll of hearts and minds, and violates the lives of those victimized by junkies who say their habits run to $400 a day, is documented forcefully, and the fact that government has absolutely no inkling of a solution to the problem becomes depressingly apparent. But watching Geraldo casually walk by an alleged drug bust being conducted in a parking lot and doing one of his call-to-conscience stand-ups, a viewer may get the feeling that star-turn reports like this one, for all the work that goes into them, aren't likely to have any more appreciable social effect than an episode of "Barnaby Jones."