If Seinfeld was a show about nothing, then Lost has been the series about everything.

The incomparable ABC drama, which ends Sunday night, spun a rich universe out of a seemingly deserted island, weaving provocative themes - faith, ethics, filicide, quantum physics, fate, and ontology, to name a few - into a compelling morality play.

"There will never be another show like this," says Marc Berman, television analyst for Mediaweek. "It started as a story of 14 people surviving a plane crash and became so much more. It's that rare breed, like Star Trek, that 10, 20 years from now you'll still be hearing about."

The saga of a core group of castaways - Jack, Sawyer, Kate, Hurley, and Locke - arrived with a surreal edge. (A polar bear in the tropics?) But it became increasingly complex and mysterious, sometimes bafflingly so.

Stories jumped backward, forward, and of late, even sideways. A sprawling, colorful supporting cast was introduced, linked to one another in unpredictable ways, only to vanish or be killed.

There were heroes, villains, lovers, shamans, scientists, ghosts, phantasms, monsters, and immortals. Death was a constant presence in the looming jungle.

We surely weren't on Gilligan's Island anymore, Skipper.

"If Lost has been so successful, in large part it's because it broke a lot of television's rules," says Jonathan Gray, associate professor of media and cultural studies at the University of Wisconsin, Madison, via e-mail.

"It filmed outside the contiguous 48 states [in Hawaii], it has a large and multinational, multiethnic cast, it has an original score, it's serial, it uses transmedia and other storytelling platforms seriously, and importantly it negotiated an end-date."

Yes, Lost is going out on its own terms, another facet that makes the series sui generis. A full three years ago, Lost's brain trust, producers Damon Lindelof and Carlton Cuse, declared that the program would culminate this spring with the conclusion of its sixth season.

That 2007 announcement was made, in part, to reassure the show's passionate but restive fans that this was not some shaggy-dog fable that was being made up as it went along, but rather a planned narrative arc with a definite resolution.

Now, with the final chapter about to air, it seems almost unthinkable that Lost could end. The series has come to resemble a self-perpetuating M.C. Escher drawing, always branching off and folding back in unexpected and impossible directions. A vaulted bridge with sweeping views that comes tantalizingly close to but never quite reaches the opposite bank.

Well, tonight, for better or for worse, it will touch down.

There are two ways to explain Lost's obsessively devoted fan base: Either they are supremely discerning pop-culture junkies. Or they are gluttons for punishment.

Because Lost is the most demanding, high-maintenance appointment viewing in the history of the medium.

"It really expects a lot from its audience, which is a huge thing for television," says Nikki Stafford, author of the Finding Lost book series. "This is a mainstream network show that says, 'What do you know about physics? What do you know about philosophy? What do you know about world history? You better go study, because we're going to test you.' "

"Lost showed that we don't just like clipped, quick stories that resolve in one sitting," says Gray. "Instead, there's tremendous appeal for some in a story that keeps going, with twists, turns, mysteries, revelations, character development, and so forth along the way."

"What Lost did for television is it proved the value of taking risks," says Berman, "of being creative, of trying something different."

It was certainly a show suited to its time, a perfect product for the digital age.

There have been inscrutable cult series before, such as Twin Peaks and The Prisoner, but they never generated the type of overwhelming buzz that Lost has.

Each episode unleashed a flood of instant online reactions, with theories echoing and amplifying around cyberspace. There are elaborate and erudite websites devoted exclusively to micro- and meta-analyses of the show.

Indeed, Lost is a virtual phenomenon as much as a TV sensation. A recent report ranked it first among all TV shows in terms of social-media interactions (mentions and reads). That indicates that the show's cultural impact far exceeds its ratings. This season, Lost barely cracked Nielsen's Top 30.

Cuse and Lindelof see the big online imprint as one of Lost's most significant legacies.

"It feels like the overall effect of Lost has been that networks are now more open to the idea that a show may air for an hour on a Tuesday night, but there can be a watercooler effect that rolls into the Internet and other media platforms, enabling new kinds of content which engage core fans at an entirely different level," say the producers jointly via e-mail. "We also hope it proves that there can be a large audience for a complex show, that you don't need to dumb TV down to the lowest common denominator."

Some fans think the show may have gone overboard in the complexity department, losing its identity while striving for profundity during its last lap.

"Lost seems to have forgotten the importance of good characters and good dialogue and good fun," says Susan Mackey-Kallis, associate professor of media and culture at Villanova University, via e-mail.

"The show was at its best when it would pay attention to small, excruciatingly funny details, like Sawyer reading Are You There God? It's Me, Margaret for lack of any better reading material, peering through badly patched spectacles that made his handsome face look adorably goofy.

"Or who can forget," she continues, "the episode where they found the VW bus and Hurley and friends went on Mr. Toad's Wild Ride, accompanied by great '70s music, as they careened across the island with no brakes on a mission to save the world! The show began to take itself too seriously, and that's always a problem!"

The penultimate episode that aired Tuesday seems to point toward a grim denouement.

Jack, with his savior complex, has assumed the mantle of the island's protector from the soon-to-expire (again) Jacob. That means he has to stop the Man in Black, the evil remnant of Jacob's brother, and his minion Ben from snuffing out the wellspring of light in the secret grotto. (Even a simple summary from this show gets byzantine in a hurry.)

Lost fans aren't expecting or even hoping for a tidy ending that will resolve the series' manifold mysteries.

"We're not going to get all the answers," says Berman.

Instead, they'll be curiously content to leave this journey just as they joined it: not at all sure where they stand.

In other words, well and fully Lost.