Well, there's no turning back now. Going forward is your only option. Otherwise, all is lost.

Season 5 of "Lost," one of television's most maddening but most addictive and creative serialized dramas, starts tonight with a one-hour retrospective "clip show" covering the first four seasons (judiciously, one would assume), then the two-hour premiere of the new season. Let's just say this much for starters - Season 5 is crazy complicated.

And yet, if you're an ambitious and eager person, you could watch the one-hour clip show for helpful clues, and perhaps comb the Internet, starting with the "Lost" page on Wikipedia and then perhaps googling "Lostpedia." It might be a more efficient use of your time to study quantum physics in your off hours, however.

It's sure not trite

That should not be construed as anything remotely negative toward "Lost," one of the few challenging series on mainstream television. It doesn't spoon-feed the American public a trite, easily predicted ending. Its ambition and creativity - and yes, the confusion - are what fuel the basic greatness of "Lost." It's just that this is a particularly bad time to jump on the bandwagon as it's rolling along. Rent or buy the DVDs and start from the beginning? Yes, by all means. Tuning in tonight expecting to know what's happening is like stumbling into a Latin class when all you're really capable of doing is texting on your phone.

One of the myriad intriguing things about "Lost" in Season 5 is that it treats the flashbacks and newer flash-forwards as, well, flash cards. Almost from the time the first frame comes into focus until the final credits roll two hours later, "Lost" takes place in a realm of shifting space and time, the after-effect of Ben (Michael Emerson) turning that super-cool wagon-wheel thingy and making the island disappear at the end of Season 4. Now, for those people still on the island, space and time are in your face and out of your mind. Just go with it. There's joy in this ride.

Roller-coaster twists

Translation: This series just took a roller-coaster hairpin of a turn. If the latter stages of Season 3 and all of Season 4 marked a course correction to the confusion that preceded it, the writers have apparently tired of making you feel comfortable in your grasp of what's happening. When Jack (Matthew Fox) says, wearily, "How did we get here? How did all of this happen?," you half expect the Talking Heads to be playing "Once in a Lifetime" in the background. Time has been "dislodged" in Season 5, and the operative words are not "Where are we?" but "When are we?" Seriously. They say that.

The essential story lines remain in place - the six people who got off the island want (and need) to go back. When we last saw Ben and Jack, they were looking at Locke (Terry O'Quinn), very much dead in a coffin in Los Angeles. Locke is alive on the island, however. And Ben is trying to put the world, or at least the island, to right. If this sounds like insanity, then you haven't been watching. If this makes sense, then congratulations on being caught up, but it's all about to go - what's the word? - insane. Season 5 starts three years after the end of Season 4. Although it seems as if every precious little hint is some kind of spoiler for "Lost" fans, and there's no joy in giving away any of the ingenious twists, setting up the structure is essential.

A lot can happen in three years. But as only "Lost" can, the series makes that point seem moot early on. There's so much time-shifting and characters from the past meeting characters from the present that time itself becomes Dali-esque.

The most impressive achievement in the first three episodes is not the various machinations of new and old characters or the mind-bending use of time. It's the parallel plotting, which is relatively easy to understand amid the motivational mental chaos. On one hand, you've got people off the island who are trying to get back on it, though someone is trying to either stop them or beat them there. On the other, you've got the people on the island (whom we are familiar with) experiencing time-space flux and meeting a lot of the island's original occupants (many we've never seen before). When these two plots ultimately meet - which might not even be this season - it's going to be a sublime migraine.

Both the producers of "Lost" and the network itself would be lying if they told you they expected to get new fans in Season 5. Stranger things have happened (see: Arizona Cardinals in the Super Bowl). But the remaining episodes of "Lost" are like the (lesser number of) remaining episodes of "Battlestar Galactica" in that they primarily are a reward for die-hard fans. These are the engrossing, final chapters of complicated mysteries. They offer only confusion to new fans. .

And yet, complicated plot twists being the mind candy of serialized dramas, they offer glorious conundrums and heretofore unheard-of turns to the loyal viewer. These Rubik's cube dramas are easy to smirk at if you're out of the loop or prefer closed-ended series where all is well in the 59th minute of every episode - and you don't need Internet annotations to follow along.

"Lost" is a different genre, one that may infuriate even the loyalists, but there's something impressive and rewarding in its density.

E-mail Tim Goodman at tgoodman@sfchronicle.com. You can read his TV blog, the Bastard Machine at www.thebastardmachine.com.

This article appeared on page C - 3 of the San Francisco Chronicle