Jack, Locke and Kate explore the mysterious hatch but Jack decides to wait before going down into the hatch. Kate and Locke aren't so patient. When Shannon chases after Vincent the dog, she encounters a familiar face in the jungle.

With the abduction of Walt fresh on their minds, their raft destroyed, and Jin missing, Michael and Sawyer fight for their lives in the middle of nowhere in the ocean and discover a new predator in the roiling sea. Meanwhile on land, Locke must descend into the hatch when Kate goes missing inside. Also, Jack isn't too far behind Locke as he decides to go into the hatch as well later on. Lastly, flashbacks reveal more of Michael's troubled relationship with ex-lover Susan and their baby son Walt as they fight over custody for Walt and Michael must let go.

Jack, Locke and Kate learn more secrets about the hatch. Meanwhile, after being beaten and taken captive, Sawyer, Michael and Jin wonder if their captors are fellow survivors or the dreaded "Others." Flashbacks reveals more of Locke's past in this episode as well as the introduction of Helen (guest star Katey Segal) who shakes things up even more with her revealing personality and past.

Hurley struggles with an assigned task inside the hatch as he flashbacks to disturbing memories in his life before the crash. Meanwhile, Sawyer, Michael and Jin learn the identities of their captors on the island. Claire uncovers a startling piece of information about the fate of the raft.

Michael sets off into the jungle by himself determined to find Walt, but discovers that he is not alone. Meanwhile, Sawyer and Jin are ordered by their captors to take them to their camp, and Sun frantically searches for her missing wedding ring. In flashbacks this episode reveals more of Jin's life before ending up on the island.

Sawyer's wound becomes life-threatening as he, Michael and Jin make their way through the interior of the island with the tail section survivors. Meanwhile, Shannon is once again haunted by visions of Walt.

The harrowing first 48 days in the lives of the tail section survivors are revealed. Flashbacks reveal the point of view of Ana-Lucia, Mr. Eko, Bernard and Libby and other survivors as they landed on the other side of the island on the beach. As the days progress danger hits their camp as well when "The Others" start taking people against their will. As the survivors race inland, trying to stay a step ahead of The Others, they realize that someone among them wasn't on Flight 815.

Violence erupts when Ana Lucia and her group stumble upon Sayid and the other castaways on the island. Ana Lucia holds Sayid captive. Kate and Jack care for Sawyer when Mr. Eko brings him back to the camp himself. Flashbacks reveal Ana Lucia's troubled life as a cop.

As Kate’s backstory continues, her original crime is revealed. She thinks something is haunting her through Sawyer. Locke and Eko make an interesting discovery about the film, and Michael has a mysterious encounter with the computer.

This episode will shed more light on why Eko took a mysterious 40-day vow of silence, and possibly information on his stick. Also, Claire begins to lose faith in Charlie after Eko begins to question him about the Virgin Mary statue, and Kate gives the recovering Sawyer a much-needed haircut.

Jack, Locke and Sawyer follow after a determined Michael after he heads into the jungle toward the dreaded "Others" in search of Walt. Meanwhile, Sun has a surprising reaction to Jin's desire to join the search party, and Hurley and Charlie commiserate over the age-old conundrum of "what women want."

When Charlie's vividly surreal dreams lead him to believe Claire's baby, Aaron, is in danger, Locke suspects Charlie may be using again. Meanwhile, Sawyer encourages Hurley to act on his attraction to Libby.

Some of the island survivors fear that "The Others" may have returned when Sun is greatly injured during a failed kidnapping attempt at her plant garden. Also, the trust barrier becomes very thin between Locke and Jack in terms of locking up the guns and being kept safe. Meanwhile, Sawyer is an amused but highly interested bystander when tension escalates between Jack, Locke, Kate and Ana Lucia. When Jack purposely takes Sawyer's medication from him he pushes more in favor of helping Locke. Lastly, flashbacks reveal more of Sawyer's con artist past.

After Sun is attacked, some of the survivors begin to think that one of the "other survivors" might be one of "The Others." When French woman Danielle Rousseau leads Sayid to a mysterious captive in the jungle, he becomes determined to find out if he is one of "The Others". Meanwhile, Sawyer discovers Hurley’s potentially devastating breach of the survivors’ trust and blackmails him into helping track an elusive island creature that won’t leave Sawyer alone.

A mysterious illness that baby Aaron suddenly contracts sends a desperate Claire, Kate and French woman Danielle to return to the same area where Claire was initially kidnapped. It is there where she believes she can find a cure for Aaron's illness. In the meantime, Jack and Locke are doing their best to keep their prisoner a secret from the rest of the survivors on the island.

Sun comes to the realization that she might be pregnant. She struggles on whether to tell Jin about the situation. Meanwhile, with no success from Jack or Sayid, Ana-Lucia is called in by Locke to interrogate the new prisoner Henry Gale to get more information out of him about the balloon that he and his wife supposedly landed on the island with.

When the hatch suddenly takes on a life of its own, Locke is forced to enlist the help of an unlikely ally to control the matter. Meanwhile, Ana Lucia, Sayid and Charlie go off into the jungle to find out the truth about Henry Gale and see if there really is balloon craft out there in the jungle that carried him and his wife to the island.

Libby lends a helping hand to Hurley to support him when he begins to think the island is having a strange effect on him. Hurley begins to see Dave (guest-star Evan Handler, Sex and the City) on the island, who was his friend in the mental institution Hurley was in. Also, Locke's sense of purpose is shaken when the prisoner gives him new information about the hatch.

After finding an exhausted Michael in the forest, Jack and Kate bring him back to the main camp. When he finally wakes up, Michael has some new details about "The Others." Also, a lovestruck Hurley plans a date for Libby.

Mr. Eko has dreams about his brother leading him somewhere. He seeks Locke's help in finding a secret location, the "question mark." Meanwhile, Jack and the other survivors grapple with the horrific situation in the hatch with the death of Ana-Lucia and Libby clinging to life.

A determined Michael convinces Jack and several castaways to help him rescue Walt from "The Others." With Jack away, Locke is left in charge of the hatch and must decide if he should believe Henry and not push the button, risking everyone's safety. Meanwhile, the events that happened to Michael after he left are finally revealed. Meanwhile, Charlie struggles with Eko's decision to discontinue building the church.

After discovering something odd just offshore, Jack and Sayid come up with a plan to confront "The Others" and hopefully get Walt back. At the same time there will be an answer to the question of where Michael has been and resolution of him and Walt. Meanwhile, Eko and Locke come to blows as Locke makes a potentially cataclysmic decision regarding the "button" and the hatch. Lastly, Desmond returns and he sheds some more light on his experience on the island in the three years prior to when Locke came down into that hatch.

After discovering something odd just offshore, Jack and Sayid come up with a plan to confront "The Others" and hopefully get Walt back. Meanwhile, Eko and Locke come to blows as Locke makes a potentially cataclysmic decision regarding the "button" and the hatch.