Those persistent old conscience prickers Richard Levinson and William Link are at it again. The latest in their long line of Deeply Disturbing social melodramas, "The Guardian," premieres at 8 tonight on Home Box Office. The film, an HBO original, reunites the two writers with actor Martin Sheen, who starred in such previous Levinson-Link meditations as "The Execution of Pvt. Slovik" and "That Certain Summer."

It's really a slander to have Sheen repeatedly cast as the conscience of humanity; if only humanity could sue. In this didactic fabrication, Sheen plays Charles Hyatt, the sole decent civil libertarian in a New York apartment house full of frightened people who surrender their freedom to a security guard as an exchange for peace of mind. Louis Gossett Jr. plays the guard, and his brittle, intimidating diction serves him as well as it did in "An Officer and a Gentleman"; he bites the heads off words and spits them out again.

But what could have been a pungent thriller on the dangers of vigilantism instead turns into a specious pontification. Levinson and Link, with an invaluable assist from director David Greene, tighten the tension fairly effectively as the guardian assumes more and more control over the tenants and their lives. But just when the film should be moving into the showdown-resolution phase, the writers throw everybody a ridiculous ringer, and narrative credibility shatters.

Hyatt alone has figured out that the security guard, not a thief, ransacked a first-floor apartment, after its tenants had scoffed at the guard's warnings that their windows weren't safe. Hyatt deduces that an intruding burglar was murdered in cold blood by the guard, not in self-defense as is claimed. Hyatt figures out that the guard roughed up a lovable newspaper dealer because the man had failed to corroborate one of the guard's alibis for using excessive force. And so on.

All that sets up a climax that is frivolously squandered. Hyatt, enraged, takes a taxicab out to the guard's home in a desolate Queens neighborhood all alone, late at night, even though Hyatt himself had described him as "very disturbed." He sends the cab away and then discovers the guard isn't home. So he wanders the bombed-out streets and is naturally soon surrounded by a vicious gang, to whom he surrenders all his belongings. There's a particularly silly threat, Hyatt turns into a quivering jellyfish, and the guardian happens along just in time to rescue him.

This preposterous development facilitates an equivocating "whither society" ending that covers everything in a soppy wet blanket. The writers eschew honest, functional dramatic expediency so they can make their big point about the dangers of sacrificing freedom in the interests of safety and present the wider allegory about people capitulating to dictators. They just couldn't find a way to work out the plot and the message in satisfying or even plausible terms.

Levinson and Link also load the deck in suspect ways, starting with the crime that sets the plot in motion and causes the tenants to seek help. Teen-age boys have attempted to rob an apartment and are caught in the act by the man and woman living there. They bolt, and then the man discovers $14 was taken from his wife's purse. He pursues and attacks one young man; another, in panic, shoots and kills him. Thus, it is implied, the man is partly to blame. Most of the crooks in the film are made to seem rather harmless, except for a rapist. Thieves are just trying to get by; rapists are vermin.

Sheen seems to get shorter and more furtive with each performance. If the writers didn't conspire to kill off the possibilities in their own premise, his neurotic performance would do the trick. As played by Sheen, Hyatt's a woeful mole, burrowing himself into a sweat, and you certainly can't blame the other tenants for not listening to his warnings. If the word "wimp" hadn't been invented yet, this would be the moment.