Your last chance to get 'Lost'

As the final series of ' Lost'hits the box tonight, viewers fret over the winding down of that rare television animal – a full-blooded media event

IF YOU WANT to understand just how good Lostis, then start with an episode of FlashForward. That's another US television show featuring a mysterious and catastrophic event, time travel, a storyline that bobs between past and future, and a grand conspiracy.

It also, for what it's worth, features a couple of Lostcast members and has featured hidden clues about the other show's plot (both are aired on ABC). Why is it worth watching an episode? To see how mundane it is; how poorly paced, with clichéd characters, clunky dialogue and a central mystery that treats itself far more seriously than anyone else will. It is, in other words, not Lost.

The sixth and final series of Lostbegins tonight at 9pm on RTÉ2, bringing to an end a series that was ostensibly about a group of plane-crash survivors stranded on a desert island, but which wove layer upon layer of mystery until its viewers developed an obsession with it. It has become a major influence on modern television and a model of how to manipulate the web. And when it ends, it will be one of those rare things from television drama: an event.

If you haven't seen Lost– or if you watched the first couple of series but abandoned it during its slow second season – then you're not going to catch up in time before it returns to Irish screens. It's not a programme you get the gist of after a couple of minutes.

The story, in a nutshell, revolves around the island being the centre of some potentially catastrophic experiment by a shady group, "the others". Its executive producers, Damon Lindelof and Carlton Cuse, last week drafted a list of six episodes they think are indispensable to the final series, but they skipped two series in the process. To simply watch those six episodes, or to watch the hour-long catch-up this evening at 8pm, or an eight-minute, 15-second YouTube highlights package, would be to strip it down to the inessentials. Because the success of Losthas always been less about the story and more about its telling.

And it is, much of the time, ridiculous: a smoke monster, a wandering island, dead people who don't stay dead and people who get beaten to a pulp but keep their wonderful teeth. And it has lost viewers with each season, thanks to how easy it is to lose track of the plot and never find your way back in. But its creator JJ Abrams – who has since brought a magic touch to the Star Trekfranchise – has relied heavily on what he calls the "mystery box". You desperately want to know what's inside. You get plenty of clues. But each answer usually brings with it only new mystery.

The show originally did this by wrapping Twilight Zone-type episodes within a wider story arc, so that it operated at a couple of levels. But since then it has, to be blunt, gone bananas, jumping between past, present and future while keeping everything and everyone connected somehow.

In the process, it has become a major influence on modern television. You can see that on US television in the form of the clumsy FlashForward, in elements of Heroes, and in post-apocalyptic show Day One. On British television, there was the recent time-travel thriller Paradox.

However, it has also been at the confluence of several other trends in US television. It owes debts to trailblazers such as Twin Peaksand The X-Files(JJ Abrams's post-Lost show, Fringe, is a rip-off) and played on the success of reality TV hit Survivor.

Its complex, multilayered narrative sits neatly in an era when cable television had pioneered the "novelistic" television of The Sopranos. That it could run entire episodes in Korean, with subtitles, exploited the growing popularity of Asian cinema. That it could stretch a day's drama into several weeks owed something to 24.

But much of its success is due to the pushing of the series beyond the confines of television. Each series has become a media event. It is not the first drama to use websites as tantalising clues – that goes back to The Blair Witch Projectand AI: Artificial Intelligence– but it now sits among a vast nexus of online timelines, podcasts, discussion sites, homages and parodies. It has fanned much of this by making mobile episodes and online clips that ran when the series itself was off air.

CRUCIAL TO THIS, though, has been its exploitation of a conspiracy culture. By drip feeding clues, and occasional major revelations, it has been brilliant at giving just enough plot to leave the viewer a little more confused, sometimes a little more informed. It has always let the viewer fill in the blanks so that watching Lostbecame an exercise in writing your own story.

What was going on? No one knew. But every single detail of the plot might be important, and occasionally it would throw in a giant stump of an Egyptian god, or an ancient pirate ship or a body that washed up before the victim had been killed. The online discussions would light up. Many dots would be joined up. The pattern wouldn't always be clear. But Lost's writers understand that the fun about conspiracy theories is not in the solution but in the speculation.

If you don’t care to speculate, then you will want to block your ears for the next few weeks. If you love the show and are dying to know what happens, then the two days’ delay between the final episode being aired in the US and Ireland will involve stealthily working your way through the web. And to those who have decided to work their way through the box set, spoiler alert! It’s not just about people stuck on a secret island.

FIVE REASONS WHY ' LOST' WORKS

1 THE OPENING SEQUENCE

The title steadily moves forward. The music goes “whuuaaaaaa”. That’s it. It’s a prime example of the art of the title sequence.

2 THE UNDERLYING THEMES

The first couple of series were metaphors for the US’s fear of terrorism, as epitomised by the physical and psychological brutality perpetrated by the Others. Meanwhile, former Iraqi soldier and ace torturer Sayid was a crucial and complex character.

3 IT KILLS OFF MAJOR CHARACTERS

Aside from the will they/won’t they central duo of Jack and Kate, there was often a sense that anyone else could be killed off. Several major characters have met sticky ends, although the blackly comic deaths of cameo characters remain fan favourites.

4 WHO'S THE STAR?

While Jack appeared to be the central character at first, the show now gives several options. It was on-the-run Kate at one point, then John Locke (who seems integral to the overall mystery). But the central character could as easily be Juliet, Hurley, Benjamin Linus or Vincent the dog.

5 EVERYTHING MATTERS

It makes viewers obsessive because every detail – an item of clothing, a board game being played, a book being read – might be a clue. It drives you insane after a while.