Immediately after their Sydney-to-L.A. flight crashes on a remote tropical island, the surviving passengers — including tortured doc Jack (Matthew Fox), feral fugitive Kate (Evangeline Lilly), and quippy conman Sawyer (Josh Holloway) — buckle up for a bumpy ride: Their pilot is munched by a weird creature. They hear an eerie, 16-year-old distress transmission in French. A freakin' polar bear charges them from out of nowhere. ''Guys?where are we?'' asks washed-up rock star Charlie (Dominic Monaghan). We're still looking for the answer six years later, but the place to start that quest is here, the episode that deftly unveiled the weird world of Lost. —Dan Snierson
It turns out John Locke (Terry O'Quinn) — a.k.a. Lost's man of faith — wasn't always a mysterious, sparkly-eyed, boar-hunting man who was unafraid of the island-terrorizing monster. He used to be a sad sack, unlucky-in-love box company employee who just wanted to take a break from his miserable life by going on a walkabout in Australia. It also turns out that Locke was even more confined in his previous life than we could have imagined. Welcome to the show's first truly mind-melting moment, and a declaration: Gilligan's Island, this ain't. —D.S.
The faith vs. reason, destiny vs. free will debate embodied by John Locke and Jack Shephard explodes into near ideological warfare, framing the philosophical context for the Lost saga. The pressing issue, so to speak: whether or not inputting Hurley's cursed lotto numbers (4, 8, 15, 16, 23, 42) into an old computer and pressing a button every 108 minutes actually does something, or if it's just a twisted psych experiment concocted by the mad scientists of the Dharma Initiative. The theme of what and who can be trusted is made manifest via the Hatch's choppy, potentially dubious orientation film, which introduced a term that looms large here at the end of the series: ''The Incident.'' Locke: ''We're going to need to watch that again.'' So will you. —Jeff Jensen
The season that pitted the castaways against the Others began with the shocking reveal of how and where the alleged ''natives'' lived — and introduced the beautifully tragic/tragically heroic Juliet to the mix. Also memorable for its deep dive into Jack's redemption issues and disclosure of his shameful pre-Island secret: He knocked his alcoholic dad off the sobriety wagon! —J.J.
''THE MAN BEHIND THE CURTAIN'' (Season 3, Episode 20)
In the first full flashback episode for Benjamin Linus (Michael Emerson), we meet the scheming scoundrel's mom (who died in childbirth) and dad (a drunk who blamed his son for his mom's death); we see Ben and his dad enter the Dharma Initiative in its 1970s prime, led by a long-haired hippie named Horace Goodspeed (Doug Hutchison); young Ben (Sterling Beaumon) meets island ''hostile'' Richard Alpert (Nestor Carbonell), who we learn isn't so much with the aging; and we see how Ben (Michael Emerson) manages to rid the Island of those pesky Dharma-ites. Meanwhile, in the present, Ben introduces Locke to the Island's true leader, Jacob — who lives in a rickety cabin surrounded by black ash and isn't exactly corporeal — which Ben instantly regrets after Jacob communicates with Locke in a way he never has with Ben. All told, it's a jam-packed, mega-download doozy. —Adam B. Vary
The episode finally brought Charlie Pace to the fate Desmond (Henry Ian Cusick) foresaw in ''Flashes Before Your Eyes,'' and the heroin-addicted rocker learns the boat the Flight 815 survivors think was sent to save Desmond (and, by proxy, them) by his eternal beloved Penny Widmore (Sonya Walger) isn't what it seems at all. Throughout the episode, we see a bearded, suicidal, drug-and-booze addicted Jack in L.A., wracked with guilt over the death of?someone. But it's not until the mind-melting final scene that we realize Lost is about things far grander than merely how these people will escape from that wild-and-crazy island. —A.B.V.
Sayid (Naveen Andrews) and Desmond are en-route via helicopter to the mysterious freighter anchored off the island, when, zap!, Desmond finds his consciousness hopping between his present and past in 1996, further plunging Lost down the time-travel rabbit hole. As genius physicist and freighter-folkie Daniel Faraday (Jeremy Davies) explains to Desmond — that is, in 1996 — hopping through time is a nasty side effect of off-Island travel, and a deadly one, too, if you can't find a ''constant'' from both time periods to ground your mind in the here and now. When Desmond finds his (one guess who it is), it's arguably the most wrenching, powerful, three-hanky-minimum scene in Lost history. —A.B.V.
''THE LIFE AND DEATH OF JEREMY BENTHAM'' (Season 5, Episode 7)
The intriguing saga of John Locke widens and deepens as he's charged with heading back to civilization to persuade his fellow castaways to return to the Island. When Ben interrupts a distraught Locke right as he's about to hang himself, a chilling turn of events follows, and the grave consequences extend all the way into season 6. —D.S.
Trapped in the past and facing certain death, Jack and his fellow quantum leapers conspired to destroy the Island with an H-bomb, thus creating a history-smashing paradox that could rewrite their destinies. Meanwhile, we learn that the Island is home to two ancient, seemingly supernatural figures who are at odds with each other: Jacob, the god-like director of the Others, and the shape-shifting Man In Black. Posing as...someone, MIB manipulates Jacob's former golden boy Ben to...oh, but why spoil it. Go watch it already. But hurry: The end of Lost begins Feb. 2. —J.J.