THOSE WHO made "Roots: The Next Generations" have achieved the nearly impossible and the extremely unlikely: They have improved on a triumph. The new "Roots," offered as a "continuation" of the Alex Haley family history begun in the first, is a more polished and sophisticated production, has more dramatic depth, and could be an even more invigorating experience for however many millions choose to watch it.

ABC insists on advertising "Roots 2" as both a "novel for television" and as "a true story." Just how true or untrue it all is we will never know, but there is an essential and moving truthfulness about it that justifies whatever may have been fudged.

The sequel does require a substantial commitment of viewing time; whether the estimated 130 million who saw some or all of the first "Roots" will be willing to schedule another entire week around it and produce more epochal ratings remains to be seen. The 14 hours air in two-hour chapters starting tonight at 8 on Channel 7, continuing Monday at 8; Tuesday, Wednesday, Thursday and Friday at 9, and concluding next Sunday at 9.

The reason ABC is skipping Saturday is so that "Roots 2" can appear in two different Nielsen weeks; a ratings week ends with Sunday, and unlike the first "Roots," this one has been scheduled during a crucial "sweep" month. The last episode of "Roots" drew 71 percent of the viewing audience the night it was shown, but the competition consisted of stumblebum crime shows on NBC and CBS. This time the other networks are combative. Tonight "Roots 2" premieres against two big-league, first-run movies: "American Graffiti" on NBC and "Marathon Man" on CBS.

But after seeing the first four chapters and excerpts from the remaining three, it really does seem safe to say that in a way there is nothing on television this week but "Roots." The sequel is a stirring, absorbing and inspiringly performed epic, momentous in the liveliest possible sense and reverberant with echoes of Americana -- hot and cold, beautiful and horrible.

It takes the Haley story as he traced it from 1882 (12 years after the first "Roots" ended with the promise of freedom for Chicken George and his family) to 1970. Thus it gets into much more sensitive areas of race relations in the United States than did the first, when every racist carried a whip and the subject was slavery.

But while the new "Roots" is more complex and more contemporary than the first, it is still so strongly a family's story, and such an infectiously affirmative one, that it is improbable that viewers will head for the hills when "Roots" starts hitting close to home. Home is the essence of the thing. And to assume that people will tune out at the first sign of bad news in prime-time entertainment and cast off for Fantasy Island is to emulate the thinking of certain pudding heads in network executive suites who have brought TV to its current state of seldom-interrupted patty-cake mediocrity.

The first chapter deals with, among other things, interracial marriage and the animosities it may provoke or have once provoked. The second deals with the disenfranchisement of blacks through skullduggery in southern legislatures near the turn of the century. The fourth depicts, probably for the first time on TV, the plight of the black soldier who fought in World War I only to return to a democracy made unsafe for him. In the last chapter, Marlon Brando, who volunteered his services for a small role on the condition that he play "a heavy," appears as latter-day demagogue George Lincoln Rockwell, whom Haley interviewed during his career in journalism.

Always at the heart of the story is the progress of the family and its offshoots and the little town of Henning, Tenn., that was recreated in Los Angeles for the film. Here the best of times and the worst of times take their toll on everyday lives. In chapter one, the first train ever to stop in Henning puffs proudly up to the station and a crowd's cheers. But there is another side to this Norman Rockwell scene. The train station's rest room doors have been freshly painted "white" and "colored," and blacks who attempt to board the train are directed to the "Jim Crow car."

"Roots 2" is not a replay of hatred's greatest hits, however, and some may feel not that it shows too much shameful history, but not enough. In part two, a black man is chased from the town by a mob, tied to a tree and burned alive, but the burning is merely suggested and we hear not so much as a scream. Instead the camera pans the faces of the lynchers. The impact has less to do with the horror of the event than with the fact that it takes place within the context of a story about people we believe in and respect. The secret of "Roots" is the oldest in fiction. We can't wait to find out what happens next.

"Roots" may be the best and most influential showboat melodrama ever produced. "Roots 2," like the first "Roots," repeatedly invokes the elemental themes and ideas that matter most: life, death, family; beauty, honor, truth, duty; freedom, dignity, love, hate, fear, trust, pride and courage. The sequel captures the sadness of farewell and the joy of reunion. It exploits one's emotions with enviable skill; the closing shot in chapter one is a tremendously affecting visual epitaph to Chicken George, whom we have come to regard as family of our own.

ABC senior vice president Brandon Stoddard, the executive in charge of "Roots" 1 and 2, wasn't just "whistling up a hollow tree," to quote the script, when he said from Los Angeles this week, "I don't think 'Roots' is like any other television show. There is an involvement and a relationship with the audience which is extremely deep. I think it has to do with what it says to them about themselves, their families, their world and their country."

Stoddard was asked if he gets choked up at the more goose-pimply moments in "Roots." He said, "'Wiped-out' is more like it. There are times I just plain start crying."

The producers and writers were sure to include a couple such moments in every chapter. One guaranteed heartmelter occurs whenever a baby is born and, true to the tradition begun in Africa by the father of Haley's ancestor Kunta Kinte, the child is taken out into the night and held aloft to behold "the only thing bigger than yourself." In part two, Georg Stanford Brown as Tom Harvey takes his grandson out to the moonlit night. In part four, the child held aloft is Alexander Murray Palmer Haley.

"Roots" was made doubly indelible by the large number of actors it introduced or gave a fortuitous new exposure. The same kind of passion has gone into the performances in the sequel, and this time the young performers likely to make the strongest impressions are women -- Debbi Morgan as Tom Harvey's daughter Elizabeth, Irene Cara as Bertha Palmer (the mother of Alex Haley), Fay Hauser as Carrie Barden, and Bever-Leigh Benfield, whose romance with Stan Shaw, as Will Palmer, is enacted with charm and poignance.

Irene Cara and Dorian Harewood, as Simon Haley, make the handsomest couple in the series. As Haley's parents, they may have been written too good to be true, but they look so fine together that this can be overlooked. There are some older actors who don't exactly get lost in the crowd, either, including Henry Fonda as the fencedancing racist politician Col. Warner, knocked for a loop when his son, played by Richard Thomas, falls in love with a black woman in tonight's opener.

Avon Long, the champion trouper who once played Sportin' Life on the stage, assumes the Chicken George role initiated by Ben Vereen. Harry Morgan shows surprising Vigor as the drunken proprietor of a lumber yard. In the third chapter, the two most commanding figures are Ruby Dee as a Haley family matriarch and Ossie Davis, magnificent as a Pullman porter who teaches young Simon Haley about the job. For his 22 years of service to the railroad, the old man is fired because a company spy tricked him into merely discussing the idea of union organizing.

It seems inevitable that the careers of Shaw, Hauser, Morgan, Banfield, Cara and Harewood will blossom after their exposure in "Roots: The Next Generations."

And Georg Stanford Brown, who appears in the first two chapters and directed the fifth, again proves himself a strong and assertive young actor. How bizarre that he should have had to squander two or three years as a regular on "The Rookies."

Supervising writer Ernest Kinoy and fellow writers Sydney Glass, Thad Mumford and John McGreevey based their scripts on notes Haley dictated to elaborate on a span of time covered in only 30 pages of the original book. Directors John Erman and Charles S. Dubin, producer Stan Margulies and executive producer David L. Wolper have obviously sensed and survived the mandate to follow "Roots" with a continuation at least as good and probably better.

In a way, they were the caretakers of a national trust, and it now appears certain that they have taken the greatest care possible; touches of hokiness or lapses in the quality of dialogue are simply crushed in the path of the ongoing saga.

As for ratings, Stoddard, 41, said he does not want to make any predictions. "I know this is going to sound like a load," he said, "but I haven't projected anything. My thinking about shares and ratings is so mixed with hopes and fears that I couldn't be accurate. Our goal was simply to make it as good or better than the first one, because if we didn't, we'd be accused of perpetrating the greatest rip-off of all time."

Stoddard admits that not everyone at the network knows what a treasure it has in "Roots" and how important it was to follow it with a sequel worthy of the original. "From time to time, people would come in and say, 'Can we do "Chicken George Meets Bigfoot?"' or a 'Roots' Ground Hog Day Special," Stoddard recalls. "But [ABC TV president] Fred Pierce said all along, 'If you're going to do it, do it the same way.' We all live through the 'Rich Man, Poor Man' experience. When they told me that show was going to series, I thought, 'My God -- 22 hours in five months!' It had taken us three years to do the first 12 hours."

Even though "Roots 2" comes to a present-day conclusion, as Haley (James Earl Jones) returns to Africa and finds the link to his heritage, one would expect of a TV network that it will find some way to do a "Roots 3." But Wolper has said, "No. With 'Roots: The Next Generations' we will have finished the story." And Stoddard said at a press conference in Los Angeles, "Believe me, we'll end it." He said this week that the network would rerun "Roots 2" no sooner than 18 months after this week's showing.

"Roots 2" does ask a lot of time and attention, but the challenge of seeing it through to the end is the same kind of challenge you get from a big, multigenerational classic novel. "Roots 2" becomes a classic almost the minute it gets on the air, and it also becomes immediately a part of the national memory.

As Tom Harvey, Brown tells his children early in the first chapter, "Them stories from faraway times, they's all true," and so we believe them to be if not literally true than filled with truth. "Roots 2" is about the importance of remembering, and fittingly enough, it promises to be genuinely unforgettable.