Hurley realizes that the numbers on Danielle Rousseau's map resemble his winning lottery numbers and sets out to talk with her. Locke gets Claire to help him make a crib for the baby.
48 people survive a horrific plane crash and begin to explore their jungle surroundings and scavenge what they can from the wreckage of the plane; several survivors encounter a creature in the jungle.
The 48 survivors scavenge from the plane the items most useful for their survival. Led by Jack and Kate, the group band together and try to protect themselves from the harsh weather and jungle terrain. Ominous howls from jungle sets everyone on edge.
Jack and Hurley learn that Kate has a disturbing past and was actually in custody of the marshall whose life now hangs in the balance. Meanwhile Kate, Charlie, Sawyer, Sayid, Boone and Shannon worry that telling the other survivors what they have discovered on their exploration of the island will cause panic.
The survivors are given a scare when wild beasts invade the beach encampment at night; the group learns more about Locke.
In an effort to save a survivor from drowning, Jack must save Boone from his own ill-fated rescue attempt and the original victim drowns. The guilt from this failure and other issues from his past prevent him from sleeping and cause him to suffer from delerium, revealing what brought him to Australia and shed light on his relationship with his late father. Claire's health takes a turn for the worse just as the last of the water is stolen.
Jin and Sun witness Michael being brutally beaten. Katie, Jack, Sawyer and Sayid disagree about where the survivors should set camp.
Locke helps Charlie through a painful withdrawal from his drug addiction. The Survivors struggle to free Jack who is buried in a cave collapse.
After Shannon suffers an asthma attack Jack and Sayid try to forcefully convince Sawyer to share his medicine.
Hurly comes up with a wacky plan to bring civility to the island. Sayid's life is threatened when he determines the source of a French transmission.
Charlie, Jack, and Kate worry that Claire's nightmares are coming true, possibly threatening both her and her baby's life.
Jack's inner demons come back to haunt him. A rescue party searches the jungle to find Charlie and Claire.
Kate and Sawyer find a metal case containing an item that she is willing to hurt Sawyer and lie to Jack to get ahold of: a toy airplane from the man she loved and killed. Kate remembers the bank robbery she masterminded. Rose helps Charlie to deal with Claire's abduction.
When Boone threatens to reveal the details about the metal door he and Locke found, Locke ties him up and drugs him. In his drugged state, Boone remembers his past with Shannon, who is actually not his biological sister, but a step-sister.
Michael remembers how his girlfriend left him and moved away, without letting him spend time with his son Walt. Michael's relationship with Walt continues to crumble, especially after Michael threatens to kill Locke if he goes near his son once more. But Michael soon has to apologize to Locke to help him rescue Walt from danger. Charlie takes possession of Claire's diary for safekeeping.
Claire returns to the compound with no memory of what Ethan has done to her or why he wants her back. Ethan injures Jin and tells Charlie that he will kill one survivor a day until Claire is returned to him. Locke and Jack come up with a plan to capture Ethan, but Charlie arrives and shoots kills Ethan before they can find out why he was on the island or what his intentions were.
Sawyer comes to believe that a boar has singled him out to terrorize. As he and Kate hunt the boar, Sawyer is plagued by memories of being set up to murder a man that he was convinced caused his parents' deaths. Hurley and Sayid worry that Charlie is feeling intense guilt for killing Ethan.
Michael accuses Jin of burning the raft he was building when Jin suddenly has burns on his hand. During a confrontation between Michael and Jin, Sun stops the fight in English, shocking her husband and the others. Jin leaves Sun, tortured by memories of who he became while working for her father. Sayid asks permission to date Shannon from Boone.
Boone becomes concerned when Locke has trouble walking while they try to find a way into the mysterious hatch. But Locke is more concerned by memories of his parents and unlocking the cryptic visions he sees which leads them to a crashed drug-runners plane.
Jack tries to help save Boone's life with help from Sun, but just as he is about to cut off Boone's destroyed leg, Boone requests that he be allowed to die. Shannon, unaware of her brother's wounds, enjoys a romantic evening with Sayid. Claire goes into labor and has to deliver the baby with Kate's help since Jack is preoccupied with Boone.
Boone's funeral leads the remainging survivors to grow suspicious of each other. Jack, Sayid, and Shannon all look for answers from Locke about Boone's death. Sawyer turns out to be just the thing to calm Claire's unhappy baby. More is discovered about Sayid's past.
When Kate goes after Sawyer's seat on the raft suspicions fly when Michael is poisoned. Flashbacks reveal painful secrets from Kate's past including the origin of the little toy plane and how she was responsible for the death of its original owner, her childhood sweetheart.
Rousseau arrives with warning about "the others" as Michael and Jin prep the raft for sailing. The monster reappears and attacks, but Locke is able to disarm the "security system," as Rosseau calls it. The people who are going on the raft set off while the remaining 40 survivors plan to blow the Hatch and hide.
Danielle Rousseau kidnaps Claire's baby for the "others," but he is recovered by Sayid and Charlie. Claire decides to name him Aaron. The raft is attacked by the "others" and it turns out that Walt, not Aaron, is the child they were searching for. They kidnap him and set the raft on fire. The hatch is finally blown, but what is inside only raises more questions. And there is one final glimpse of the monster.