An AIDS movie with a happy ending would, at this point, have to be a lie. In "An Early Frost," the NBC film at 9 tonight on Channel 4, a doctor tells the mother of an AIDS victim, "Allow him his hope. It is the only weapon he has left." The movie allows the audience some hope, too; the last time we see the AIDS victim he is smiling, and telling his parents he loves them. Still, the filmmakers can't reasonably be accused of having sugarcoated an agonizingly bitter subject.

NBC took pains to point out that the film was in the planning and production stages for two years, begun well before the publicity surrounding the late Rock Hudson, so far the most famous AIDS victim, surfaced. Even without such assurances, it's clear the film is not cynical exploitation.

Indeed, it seems an agile balancing of dramatic values with social responsibility. Although the movie will be followed by an NBC News special on AIDS at 11:30, the film itself has obvious informational roles to play. TV movies have a strange validating function when it comes to topics like this; everyone has read the headlines, seen the news reports, and now a TV movie is called for to help adjust focus. The producers say they tried to keep the medical information up to date; it is not likely that the film, with its reasoned tone, could contribute to what sometimes seems a wave of hysteria regarding AIDS.

The writers, Daniel Lipman and Ron Cowen, and director John Erman seem to have been careful every conceivable step of the way. To avoid the wrath of a highly offendable constituency, for example, the central character has been made so far removed from the stereotypical homosexual that it could be argued he is stereotypically unstereotypical. Where the script hesitates or treads very softly, though, the actors and their command of craft make up for any related loss in credibility. They convince us that this really is a story about people and an unnervingly immediate ordeal.

Aidan Quinn, who earned heartthrob status in films like "Reckless" and "Desperately Seeking Susan," plays Michael, the promising young lawyer just promoted to partnership in a firm, who learns about 15 minutes into the picture that he has acquired immune deficiency syndrome. The essence of the film is how members of Michael's family react to the news both that he is gay and that he has AIDS.

The mother, played with glowing sensitivity by Gena Rowlands, struggles to adjust, while the father, played by Ben Gazzara, feels betrayed -- by the lie he feels his son has been living, by the disease that now threatens to kill him. Both Gazzara and Quinn underplay masterfully; this is a case where a minimalist approach produces considerable impact.

While actors playing doctors dole out the cold medical facts, or theories, about AIDS, the great character actress Sylvia Sidney, as Michael's grandmother, becomes the source of compassionate wisdom. "It's a disease, not a disgrace," she lectures.

There are other reactions. Michael's sister Susan (Sydney Walsh), who had prided herself in her liberalism at accepting her brother's homosexuality, retreats into fear for herself and her children, born and unborn, when she learns he has AIDS. When, at home, Michael suffers a seizure related to a brain infection, paramedics responding refuse to transport him to the hospital.

All this corroborates many published news stories. The filmmakers have taken it upon themselves to disseminate what is thought to be clearheaded thinking about AIDS and what it represents; some viewers may quarrel with some of it. The film is on safer ground as the story of a family traumatized by a uniquely contemporary terror. At this level, the movie is about rallying in the face of crisis, but it also touches on all the hopes that parents have ever had for their children.

Not only because this has proven a timid and lulling prime-time television season so far, "An Early Frost" is likely to be the most important TV movie of the year. 'Saturday Night Live'

NBC's "Saturday Night Live" returned for what was loftily referred to as its "second decade" Saturday, and disappointment was the order of the night. The debut program, which one might have assumed would impressively showcase the new young cast, gave almost none of them a chance to shine. Much of the writing was forced or feeble, and the whole thing looked creaky and rickety.

Hoping perhaps for "Babes on Broadway," a viewer instead got "Bombs Over Brat Pack."

Madonna, queen of the known universe, was guest host, and she worked hard in a number of skits, but you knew that addled heads were in charge when announcer Don Pardo opened the show billboarding "musical guests Simple Minds." Who needs a musical guest, especially another ravingly mediocre one, when Madonna is the host?

A generation gap exists between the writer-producers and the new cast, whose youngest member, Anthony Michael Hall, is 17. The impression given by the premiere was that a coven of fogies had conspired to foist hateful or sodden mirth on their youthful charges, taking all the fight out of them. The boyish brio Hall has shown in his movies was sedated into a mere lurk (that did put him one up on the studio audience, which appeared to be watching the whole thing in a rapt trance). Only Danitra Vance, a most promising newcomer, managed to break away now and then from the constraints.

On the other hand, the writers tried to be irreverent in the old "SNL" way and that seemed either quaint or disgusting. A sketch called "National Inquirer Theater" that was supposed to be an apotheosis of lurid tabloid preoccupations ended with an actor playing John Kennedy suffocating Marilyn Monroe (Madonna) with a pillow. The "Weekend Update" segment included the requisite laugh-at-death gag: "This just in. The last remaining section of Orson Welles has died." It was funny when it was Franco.

Of course, when everyone else on television is trying to be tasteless, true tastelessness does become a challenge. It's a piece of bait the "SNL" writers ought to be sophisticated enough to resist in the future.

Dennis Miller, who did the "Update" segment, was accomplished and alert at it. Nora Dunn, who hosted the "Theater" piece, seemed to be doing a funny parlay off both Penny Marshall and Tyne Daly. But another new cast member, Terry Sweeney, was so nervous he became nerve-racking, and some of the other novices cast great shadows of gloom where one might have hoped for a tiny peep of playfulness.

Highlights included what seemed a beer commercial but which turned out to be a cleverly ominous moral edict about going to hell. A mock trailer for a movie called "Critic" included a funny, hammy cameo by aging bad boy John Simon. Penn and Teller, the brilliant magicians, were up to their new, and very original, tricks. An attempt to portray the visit of Prince Charles and Princess Diana to the Reagans as a variation on "Who's Afraid of Virginia Woolf?," however, failed for two reasons: lousy writing and lousy execution.

Randy Quaid, apparently the oldest member of the new group, stood out like a chaperone at a slumber party; worse, and like some of his colleagues, he didn't seem at all happy to be on board. Even reliable director Dave Wilson seemed beset by indifference.

Lorne Michaels, the executive producer, had only nine weeks to bring all this together -- not long in TV terms at all -- but then he had five years experience as "SNL" producer during its heyday. Saturday night's premiere looked like the old Michaels show on an off night. You remember -- those nights when the clock would strike a mere midnight and here you thought it was already a quarter to 1.