A principal emotional appeal of fantasy or space-opera films, it has been said, is to nostalgia.

At first glance, it would seem a paradox to say that stories generally set in the distant past touch an emotional yearning for the present or future. But the statement hits upon a odd truth whose equally paradoxical corollary is that no one is more nostalgic than teenagers, who over the years have been the most fervent fans of science fiction.

In both cases, the nostalgia often fixes itself on a past that never was.

For example, the two main science-fiction writers of my youth -- and I was a nut for science-fiction stories -- were Ray Bradbury and Robert Heinlein. Both created futures that were romantic, perfected visions of the American past.

For Bradbury, paradise was a peaceful Midwestern village at the turn of the century, and he managed to plant one, complete with ice-cream parlors and band concerts at sunset, on Mars.

Heinlein romanticized the rugged individualism of late 18th- and 19th-century America, the tough frontier explorers and capitalist robber barons, and he, too, took his vision of the past and thrust it into the future. This time, the frontier was space, and a big reason to explore other worlds was that they contained unimaginable wealth, like the hills of California in 1849.

George Lucas grew up reading the same science fiction I did, and he certainly knew that nostalgia was an important part of the mix. He began his first major science-fiction movie, ''Star Wars,'' with the legend ''Long ago, in a galaxy far, far away.''

Lucas took his cue not from contemporary science fiction but from the ''space opera'' of the 1930s and early 1940s. He was obviously on to something. By 1977, the youth culture that had exploded in the late 1960s had grown old, weary, commercialized and devastated by drugs.

Looking back at ''Star Wars,'' it's now clear that its focus on simple, sturdy old-fashioned values like heroism and personal loyalty, and its against-all-odds optimism, were a signal of the dawning of a new era.

You could say that the box-office success of ''Star Wars'' was an early sign that, three years later, Americans would choose as their president Ronald Reagan, a man who appealed to those old values and who made the country feel good about itself.

Lately, there have been a number of articles blaming George Lucas for the fact that modern Hollywood movies stress fast action and explosive special effects over believable and compelling plot and dialogue. He is also being blamed for creating the monster of spin-offs and tie-ins: toys, video games and the like.

Lucas is certainly good for the latter charge -- he chose to keep the rights to sequels and tie-ins in lieu of a payment of half a million dollars, and reportedly his ancillary profits to date from ''Star Wars'' are well in excess of $1 billion. If you have young children, you know what ancillary marketing, invented by Lucas and carried even further by Disney, has meant to your pocket book and your peace of mind.

But the other charge is, at least in part, unfair. ''Star Wars'' is not all fast cutting and wham-bam action, not all special effects. Despite some hokey dialogue (Harrison Ford complained that Lucas could ''type'' the words, but that didn't mean actors could say them), ''Star Wars'' does have a comprehensible and well-developed story. It should. Lucas got it from excellent sources.

Lucas pored over Joseph Campbell's ''The Hero With a Thousand Faces,'' about mythic archetypes that appear and re-appear in the stories people have told themselves throughout history. Lucas clearly mined the King Arthur legends, and took a long look at more recent, near-mythic tales such as ''The Wizard of Oz.'' (The Tin Man-like robot C3PO acknowledges the ''Wizard of Oz'' debt toward the end of ''Star Wars,'' when, damaged by hot battle, he murmurs squeakily, ''I'm melting!'')

There's a lot to be said for stealing from such time-proven tales of heroism as King Arthur, ''The Wizard of Oz,'' and perhaps ''Casablanca'' (like Humphrey Bogart's Rick, Harrison Ford's Han Solo appears to be interested only in his own welfare until it comes time to make an existential choice to do the right thing.)

Contemporary special-effects extravaganzas, including, it appears, upcoming blockbusters like ''Volcano'' and ''Titanic,'' seem mainly to steal from cheesy 1970s disaster movies, and plots are secondary to loud explosions, MTV-style shock editing and fast chases.

''Star Wars'' has a classic heroic plot, as a young man from an out-of-the-way planet heads for the center of the galaxy, bonds with other brave men and defeats an ''Evil Empire.'' It also has a wonderfully tough heroine: ''Into the garbage bin, Flyboy,'' Princess Leia snaps at Han Solo, and that's where he ends up.

Only toward the end, with ships battling in space, does fast editing take over. Compared to the sometimes incomprehensible ''Mission: Impossible,'' ''Star Wars'' is a model of linear storytelling.

With or without the new additions and improvements, ''Star Wars'' holds up beautifully. It is, not really a great movie, but it's enormously entertaining. Only in retrospect do we also realize that it is also a significant social and historical document, a window to the changing spirit of its times.