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.

As Kate and Sawyer journey back to the main island with Karl, who is exhibiting effects of the brainwashing video, they argue over whether to go back and save Jack. On Hydra island, as Juliet's fate rests in his hands and a power struggle within the Others ensues, Jack recollects the time he spent in Phuket, Thailand and his relationship with Achara, a mysterious tattoo artist.

Violence ensues and a mysterious island beast makes a re-appearance when Michael and Locke clash over Walt's upbringing. Meanwhile, Charlie is tempted to read the missing Claire's diary, and Sayid enlists Shannon to help decipher the French woman's map.

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.

Kate needs to get information out of the hostages, but it may jeopardize her standing with Locke -- as well as with Sawyer. Meanwhile in the future Kate's Court hearings take place.

Walt and the others are shocked when Michael is brutally beaten, but only the non-English-speaking Jin and Sun know the truth behind the attack. Meanwhile Kate, Jack, Sawyer and Sayid argue about where the survivors should camp -- on the beach, where they're more likely to be seen, or in a remote inland valley where fresh water abounds; and Locke discovers Charlie's secret.

Jack, Kate and Sawyer fight over possession of a newly discovered locked metal briefcase which might contain insights into Kate's mysterious past. Meanwhile, Sayid asks a reluctant Shannon to translate notes he took from the French woman. A rising tide threatens to engulf the fuselage and the entire beach encampment, and Rose and a grieving Charlie tentatively bond over Claire's baffling kidnapping.

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.

Jack suspects foul play when Michael becomes violently ill while building the raft. The suspects include Sawyer and Kate, who compete for the last seat on the raft and do anything possible to prevent each other from getting it. Meanwhile, a secret from Kate's past is revealed to the other survivors on the island.

Juliet receives an unwelcome visit from someone from her past and is given orders to track down Charlotte and Faraday in order to stop them from completing their mission -- by any means necessary. Meanwhile, Ben offers Locke an enticing deal.

Kate and Juliet must learn to work together when Jack's health is seriously compromised, and something goes wrong as Sawyer, Claire, Aaron and Miles continue their trek away from Locke's camp and back to the beach

Charlie begins a painful journey of withdrawal from drugs, surprisingly aided by Locke, whose true motive for helping Charlie is a mystery. Meanwhile, survivors struggle to find and free Jack when he's buried alive in a cave collapse, and someone might be secretly thwarting Sayid, Kate, and Boone when they enact a plan to find the source of the French transmission.

When Locke learns that Boone wants to share their "secret" with Shannon, Locke decides to teach him a lesson. This leads to Shannon's life being placed in what seems like peril. Boone and Shannon's dark past is revealed in a shocking backstory that recalls their relationship right before the plane crash and presages the return of the beast. Kate, who has become a confidante to the soft-spoken Sun, is puzzled by Sun's mysterious behavior and the revelation that she can speak English. A hungry Hurley must convince Jin to share his fish by making up for offending Jin by rejecting his offer of raw fish early on, or he'll continue to suffer digestive problems from the limited island diet.

After another funeral, tempers rise as the survivors' suspicions of each other grow, and an unlikely survivor vows revenge. The events that landed Sayid on Flight 815 play out as he engages Locke in a psychological game of cat and mouse to uncover the truth about the mishap that claimed Boone's life.

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.

Sayid confronts Ben's spy on the freighter, and Ben urges daughter Alex to flee Locke's camp in order to survive an impending attack.

Jack is nearly delirious from lack of sleep and struggles to overcome the haunting events that brought him to Australia and, subsequently, to the island. Meanwhile, Boone gets caught in a treacherous riptide trying to save a woman who went out swimming. A pregnant Claire's health takes a bad turn from lack of fluids, and a thief may have stolen the last bottles of water. Veronica Hamel guest-stars as Jack's mother, Margo. Also, Jack flashes back at 12 years old, to find himself on the playground in an altercation with a bully, who ultimately beats him up, and later learns a life lesson from his father.

Claire has a horribly realistic nightmare about her new baby being harmed or kidnapped. Flashbacks reveal Claire's backstory: the former boyfriend who got her pregnant, then abandoned her, and the psychic who convinced her to take the ill-fated flight that landed her on the island. Hurley is shocked and confused when he discovers that Ethan Rom (guest-star William Mapother), another plane crash survivor, does not appear on the flight manifest.

After the missing Claire returns with no recollection of what has happened since before she boarded the doomed Oceanic flight 815, Jack and Locke formulate a plan of defense against her kidnapper, the mysterious Ethan (guest-star William Mapother), who threatens to kill off the other survivors one by one unless Claire is returned to him. Meanwhile, the disappointment Charlie feels when Claire does not remember him triggers recollections of a woman he had let down in the past.

When the raft the survivors have been building mysteriously burns down, Michael is convinced that Jin is responsible for the sabotage, which only serves to escalate their rivalry. Meanwhile, Sun stuns her fellow survivors with a surprising revelation, and Boone gives Sayid a warning about his step-sister Shannon. Lastly, more details of Jin and Sun's troubled marriage are revealed through flashbacks.

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.

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.

Rose is surprisingly and vehemently opposed to Bernard's plan to create an S.O.S. signal; romantic sparks are rekindled between Jack and Kate when they trek into the jungle to propose a "trade" with "The Others"; and Locke begins to question his faith in the island.

Locke, Eko and Desmond are found scattered about the island after the implosion of the hatch. Locke receives a message from the Island asking him to fix the situation he caused. Hurley returns to the beach camp to report the imprisonment of Kate, Jack and Sawyer by the Others. Desmond has been fundamentally altered by the implosion, giving him a mysterious new power.

Locke finds out where Jacob's cabin is. Life on the freighter becomes dangerous.