Mike Finnegan is a joyful man trapped in a joyless life. His wife of 35 years has gone a tad batty, the neighborhood in which they live is overrun with junk and junkies, and he arrives at The Morning Times-Dispatch one day to find that his desk has been moved back by the coat rack, an insult compounding his recent demotion from the metro staff to the lovelorn column.

Into each life, however, some Mary Tyler Moore might fall, and into his, very luckily, it does, in "Finnegan Begin Again," the best and most serious romantic comedy ever made for cable television. A thoroughgoing delight, a perfectly swell little picture, an unapologetically upbeat piece of fluff, "Finnegan" costars Robert Preston and Moore and marks a high point for original programming on Home Box Office.

The film premieres on HBO tomorrow night at 8. It can also be seen Wednesday and five times in March.

HBO imagined the film to be competition for "Terms of Endearment," the popular and superlative theatrical hit that made its cable TV premiere this month on Showtime, HBO's chief competitor. Showtime has exclusive rights to recent Paramount films, and "Terms" is a pivotal blockbuster. Impressively enough, HBO came up with an original film that has not only terrific credentials but a distinctive style as well. It's a mainstream offbeat American romance that walks the laughter-tears tightrope with salutary finesse.

If Robert Preston had any more charm than he has, he would have to be two people. As Mike Finnegan, he is an ingratiator from the first frames, even though Mike, at 65, has turned dejectedly cynical. The lonely-hearts column he recently inherited is a disaster because he tells the moonstruck calves who write in just what they don't want to hear: that they should pull themselves together and stop behaving like ninnies.

On the bus to work one Thursday he meets Liz De Haan, played by Moore. She's on her way to a weekly tryst with a married man, indeed, the undertaker who buried her husband years earlier and who's a boring and uncouth schmo besides (one played with self-effacing shaggy alacrity by Sam Waterston). She's taken what she thought she could get, as Moore did at the meat counter in the opening credits of her television show. In Mike Finnegan she finds at first a corrosive adviser and counselor, later a true-blue commiserating pal. Later still, they go one natural and perilous step farther.

"To May, from September," he says, toasting her in a seedy bar. Invited to her house for dinner, he spouts blunt advice and she tells him, "You're tactless as hell, but you're truthful." What helps make "Finnegan" a beguiling surprise is that it's more the story of a friendship than of a "relationship." For Mike and Liz, it's anything but love at first sight -- she thinks this old man saying "Hello, you sweet thing" to strangers on the bus is a creep, and he finds her terminally self-deluded -- but slowly, without Mike's even realizing it, the tone of that lovelorn column changes. It gets starry-eyed. Mike has been bitten by the virus he thought went only for younger victims. And Liz realizes she doesn't just like the old guy.

Preston's performance is rousing, endearing, masterfully rascally. He makes Mike a complete and authentic human cuckoo. There's a rush of confessional merriment in his voice when he tells Moore, early in their friendship, after she suspects him of simply being weird, "I'm a newspaper man!" -- as if that explained everything. And it almost does. It's one of the best line readings I think I've ever heard.

Moore does a superb job, too, as the compromised Liz. There's no sitcom tweet in the mature, aspiringly assertive woman she plays, and while it's the male of this couple who's the more flamboyant, Moore does not surrender the screen to Preston. She's in there all the way.

The venerable, to put it mildly, Sylvia Sidney plays Mike's around-the-bend wife, dancing about her cluttered rooms while the telephone rings, later in the film suffering a stroke when the couple's vulnerable inner-city house is burglarized (the film was shot in Richmond). Visiting her in the hospital, tending to the messy needs sick people may have, Preston as Finnegan realizes there's something left of a marriage he thought extinct. When he does a little dance by her bedside to cheer her, she laughs so hard that she throws up.

Joan Micklin Silver, the director ("Chilly Scenes of Winter"), has previously demonstrated a humane way with gallows humor and an infectious affection for cockeyed characters. She makes the most of everything in the script by Walter Lockwood. Part of what makes a good director is the ability to know how much attention to give foreground and background details, and Silver has struck the ideal balance. The only lapse is an overly broad and self-consciously irreverent scene at a gravesite, a small unseemly stoop in a movie that otherwise waltzes tall.

Among other distinguished contributions, of particular value is the musical score by Michael Colina incorporating saxophone solos by the gifted David Sanborn. This is the kind of touch that says the producers aren't just doing things by the TV-movie book. It's what HBO Premiere Films could use much much much more of.

At this point in his career, it would be risky to say of Preston that he has never been better, because there've been too many memorable Preston performances. His face is now that of a big, blubbery lion, one who has regained whatever dignity he surrendered in Blake Edwards' meretricious "Victor/Victoria." Preston still has a dancer's physical grace -- he remains the Music Man -- and when in one scene he saunters along the street behind a kid with a boom box and begins to feel the beat in his walk, his body takes on a mischievous, sly, wish-I-were-a-kid-again (but-it's-just-as-well-I'm-not) lilt. This not only suits, but epitomizes, the whole lilting film. 'Evergreen'

There's simply not enough going on in "Evergreen" to justify the three-night run and the six-hour length of this hapless NBC mini-series, which begins tomorrow night at 9 on Channel 4 and continues through Tuesday. NBC probably has the worst mini-series and original-movie record in network television. It's impressive, kind of, the way this network keeps making the same mistakes over and over again. It probably wouldn't be making shows in color yet if RCA didn't own the company.

The star of "Evergreen," Lesley Ann Warren, was similarly featured in "79 Park Avenue," a dreadful "novel" for television that NBC produced in the post-"Rich Man, Poor Man" era of books-into-mini-series. Belva Plain's novel is, naturally, a "multi-generational saga," one that starts in 1909 and ends about 50 years later. People come, people go, and nothing ever happens at Grand Hotel. Or at the Plaza, either, which is prominently featured in an early shot.

Warren, who does have a superficial, beaming, slightly tarty appeal, plays a young Polish Jew who arrives in the promised land determined to make something of herself. She falls in love with two men, one wealthy (Ian McShane), the other first-generation poor but ambitious (Armand Assante), both Jewish. She marries the poor one but has a child by the rich one. The film follows the fates of this and other children, one of whom, Iris, is played by a tantalizing newcomer, Joan Allen.

In Part 2, director Fielder Cook gets some life out of young lovers played by Kate Burton, showing increased assurance with each role, and Tony Soper, a boyish Robby Benson look-alike (this is said respectfully). They are an attractive pair and they get a certain sizzle going, but of course, they get killed in a car crash. Part 3 was not available for preview.

Cook is really too classy a director for the mini-series format; no director can look very good, of course, when nearly everything is kept in and nearly nothing thrown out. He can't get a substantive portrayal out of Warren, but then, who could? God, maybe? Writer Jerome Kass may have been faithful as heck to the book; I just don't have the energy to find out. There is something a little too easy, though, about exposing anti-Semitism of the 1930s. It would be braver and more productive to expose anti-Semitism of the 1980s.

Woody Omens, the cinematographer, achieved a rich rosy look for the early scenes, but the edge was taken off the film's immigrant story by the CBS mini-series "Ellis Island" earlier this season. It was junk, but it zipped along. "Evergreen" simmers, bubbles and sputters out, but so slowly you could build a second Brooklyn Bridge before it's all over.