This episode is about when we realized that for every burning question that Lost answers, it would raise two or three more. How did Locke find himself on Oceanic 815? Easy! The put-upon, adventure-starved office drone decided to go on a walkabout to find himself. But hold on a sec, he was in a wheelchair when he arrived in Australia? How did that happen? This episode establishes that the show will contain many shocking revelations (now commonly known as "thumps") and also marks the first appearances of Locke's love interest, Helen, and Jack's seatmate, Rose.

Sure, it's fun and sexy that Kate is a bad-girl fugitive, but how did she get that way? Well, she killed her abusive stepfather by setting fire to his house while he slept off a bender, and bought her mother an insurance policy so she could benefit financially from his death. Kate's mother, however, turned her in to the authorities, setting off a chain reaction of run-ins with the law, the first of which she evades because of a black horse. When Kate starts seeing that same horse on the island, we begin to wonder how it got there. Bonus: This is also the episode where Shannon is buried and Michael responds to a text message of sorts from the kidnapped Walt while working in the Hatch.

When Claire's baby (pronounced "bay-bee") gets sick, Libby helps her reconstruct memories of her abduction, when Ethan Rom gave her injections and Mr. Friendly wore a phony beard. This was Lost's "Rosemary's Baby episode," as the audience volleys between believing Claire and assuming she's a hysterical pregnant lady. It is also during this episode that we meet Alex for the first time and learn that the Others are in the baby-napping business.

We loved Jorge Garcia in Season 5's "The Lie," but it was this episode that first made us question nice-guy Hurley's sanity and then question whether his sanity even matters on an island this crazy. Hurley's hallucinations of Dave (guest star Evan Handler), a man he remembers from the mental institution, raise some interesting questions: Is it all a dream? Does the island target people with specific manifestations from their lives? Can Hurley speak to the dead? This episode is also memorable for revealing that while Libby may or may not be a psychologist now, not so long ago she was a fellow patient in Hurley's booby hatch.

Although the flashbacks in Season 2's "Special" shed light on his son Walt's abilities, we prefer this Michael-centric outing for the way it connects some dots — explaining why a normally clear-thinking construction worker would gun down two people (Ana Lucia and Libby) and free "Henry Gale." What brought on this character-damning "betrayal"? The Others, still posing as the bedraggled residents of a shantytown, teased Michael with a brief reunion with Walt. They then coerced him into freeing Henry/Ben and "delivering" Jack, Kate, Sawyer and Hurley, all in exchange for his kid, a boat and freedom. Little did Michael know that he would never be free of the guilt he felt as a cold-blooded killer.

Juliet is a crafty Other, to be sure, but this episode tells us the story of how she ended up on the island, after a career in fertility research with her ex-husband (guest star Zeljko Ivanek) in Miami. It's a sympathetic portrait, as we learn that she is as much a victim of Ben's deceptions as anyone else. (Richard Alpert told her that she was applying for a job at a facility in Portland, Ore., hence the title of the episode.) On the island, Juliet's loyalty to Ben is tested when she assists Jack in removing Ben's spinal tumor. It's revealed that Ben has agreed to let Juliet leave the island, after three years, if he survives the surgery.

Check this out, all this happens in the same episode: Not only do we get crucial information about Ben's childhood and his relationships with his mother, father and a young friend named Annie, we also get our first look at Dharma Initiative operations in the 1970s, their skirmish with "the Hostiles," the Purge, and some mysterious dude in a rocking chair named Jacob. This was also the first time Ben left Locke for dead. Think about the importance of all those plot developments to the big picture.

Sniff. When scrappy former rock star Charlie learns he has to die to ensure the rescue of his beloved Claire and her son Aaron, he leaves behind a note that includes his proudest moments in life. We are then treated — via flashbacks — to the night Drive Shaft first heard "You All Everybody" on the radio, young Charlie learning to swim, Charlie bonding with his brother over a family heirloom, and being called a hero after stopping a mugging. At the top of Charlie's list, however, is the night of the Oceanic 815 crash, when he first met the very preggers Claire. Can someone pass the Kleenex?

Just when the show trained us to follow present-day stories and the flashbacks that inform them, "Looking Glass" shatters the formula. A sad, bearded, drug-addicted Jack stumbles through life after a botched suicide attempt and learning that someone has died. But who's in the coffin? In the show's final minutes — when Jack meets Kate and tells her that they need to go back to the island — we realize that the entire cleverly constructed episode has been a flash-forward that presents the wacky idea that getting off the island shouldn't necessarily be the survivors' goal. Meanwhile, on the island, Charlie's noble sacrifice makes us all cry.

It's no surprise that Sayid works as an assassin after being rescued; he always did have a ruthless streak. It is surprising, however, that the suave, globetrotting charmer is working for Ben. It's in this episode that Faraday confirms that time moves differently on and off the island. Also: We learn that Sayid kind of has a thing for blondes.

We always knew Desmond was different, but "The Constant" showed us how different. This is the first episode that incorporates neither flashbacks nor flash-forwards in its narrative. When Lapidus accidentally veers off course on the way to the freighter, Desmond, whose proximity to the hatch explosion has altered him, becomes unstuck in time, volleying between the versions of himself in 1996 and 2004. We learn that this form of time travel can cause one's brain to explode, which is why Desmond — with Faraday's help — chooses Penny as his "constant," a person who knows you in both time periods. Forget Jack, Kate, Sawyer, Juliet, Sun and Jin. Desmond and Penny's poignant, time-traveling affair solidifies their position as the heart of the show.

The on-island story of this episode is pretty straightforward. Juliet convinces Jin and Sun to leave the island for the sake of their unborn child. Over the course of the episode, Sun tells Jin (with an unwanted assist from Juliet) that she had an affair, but that Jin is the father of her baby. Off-island, things are more complicated: Jin's story of rushing to a hospital is a flashback, while Sun's seemingly analogous story is actually a flash-forward that tells the story of the birth of their daughter, who lends her name to this episode. In the episode's final moments, we learn that Sun has erected a tombstone in Korea that lists Jin's date of death as 09/22/04, the date of the Oceanic crash. What happened to Jin?

There have been plenty of episodes that demonstrate why Sawyer is such a shifty opportunist, and also why he needs to atone for his past actions. But it's "LaFleur" that shows the con man assuming one more pseudonym to become a good man when he assumes a prominent position of power within the 1970s-era Dharma Initiative. It's also in this episode that we learn that James and Juliet have, in the absence of the Oceanic 6, made a love connection as pure and as beautiful as those hand-picked flowers he brings her. Just as things couldn't appear more perfect, the members of the Oceanic 6 arrive on Ajira Flight 316, ready to wreck homes.