Forty-eight survivors of an airline flight originating from Australia, bound for the U.S., which crash-lands onto an unknown island 1000 miles off course, struggle to figure out a way to survive, why trying to find a way to be rescued.
Jack, Kate and Charlie return to the group, but the transceiver is broken. Sayid Jarrah fixes the apparatus and organizes an expedition with Kate, Charlie, Shannon Rutherford and her brother Boone Carlyle and the dangerous James "Sawyer" Ford to hike and climb a mountain to transmit a SOS. The group receives a sixteen years old distress call from a French woman in the island instead and kills a polar bear. They return to the survivor's base and decide to hide the information to preserve the hope of the group.
The U.S. marshal's assignment is revealed. Later, there is a difference of opinion between Jack and Sawyer as to how to help the critically injured marshal. Flashback to what Kate was doing in Australia. Scenes of the crash from Kate's POV. A small group sets off with a transceiver in an effort to send a mayday signal. Michael forbids Walt to speak to Locke. Locke later extends an act of kindness toward Michael that may help the father-son relationship. Sayid makes a bid to organize some island committees.
When the survivors run out of food, Locke, Kate, and Michael go out hunting for boar. Michael gets injured, so he and Kate go back. But Locke stays out hunting, and ends up running into the creature living in the jungle.
Trying to kick his drug habit with encouragement from Locke, Charlie starts to go through heroin withdrawl, struggling with his resolve. Jack is trapped in a cave-in, and Charlie becomes trapped with him when the first rescue tunnel collapses as well. Kate, Sayid, and Sawyer attempt to triangulate the position of the distress signal coming from the island. Charlie has flashbacks of his rock band, and his decent into drug abuse.
Ashamed with his behavior torturing Sawyer, Sayid Jarrah decides to leave the group of survivors and travel alone through the island, trying to find the source of transmission and map the place. He is arrested and tortured by the French survivor Danielle Rousseau and recalls his beloved Nadia back to his days years earlier in Iraq as part of the interrogation/torturer for the Republican Guard. Meanwhile, Hurley decides to get the survivors spirits up by getting up a golf course in a sunny field.
Claire begins to have reoccurring nightmares which one night result in her turning up with bloody hands, which leads Charlie to find out what is really going on. However, Jack isn't sure that anything really happened. Meanwhile, Hurley finds the flight manifest and discovers something shocking about a certain passenger.
Jack, Kate, Locke and Boone go after Ethan, Claire and Charlie. They separate, and while Jack and Kate confront Ethan, Locke and Boon find another mystery at the jungle. Flashbacks reveal more about Jack's past with his dad.
Boone Carlyle is jealous of Sayid Jarrah, because he becomes close to his half-sister Shannon Rutherford. Boone tells John Locke that he wants to reveal the secret about the hatch in the woods to Shannon. Locke hits Boone, heals his head with a medicine and fastens him with a rope in a tree, living him alone in the forest. Boone's background story reveals his complicated relationship with the irresponsible Shannon, and about why he ventured to Australia to rescue her from a bad relationship, while in the present he tries to save her from the unseen forest creature. ...
John Locke is teaching Walt Lloyd to throw knives, and Michael Dawson feels jealous with the situation. He forbids Walt to meet Locke, and recalls his romance with Walt's mother Susan Lloyd (Tamara Taylor), an overachieving executive who took the infant Walt away, and having an awkward reunion 10 years later with the uncertain and spoiled Walt in Australia. Walt becomes upset with the situation and decides to hike with his dog Vincent into the woods. At the end, Locke and Boone Carlyle find Claire Littleton wandering in the forest.
After the missing Claire returns with no recollection of what has happened since before the doomed flight of Oceanic 815, Jack and Locke formulate a plan of defense against her kidnapper, the mysterious Ethan Rom, who threatens to kill off the other survivors unless Claire is returned to him. Meanwhile, the disappointment Charlie feels when Claire does not remember him triggers recollections of Lucy, a woman he had let down in the past back in England where Charlie ruined their engagement so he could steal money from her to fuel his escalating heroin addition.
When Jin Kwon is having a serious quarrel with his wife Sun Kwon, Michael Dawson defends her. Later, when Michael's raft is set afire and Jin has his arms injured by heat, Michael and Sawyer blame him for the incident. Sun finally reveals her secret when she speaks in English to defend her husband and he feels ashamed leaving her. Jin recalls how his love for Sun was destroyed by her evil and unscrupulous father.
When Hurley becomes obsessed with the French woman and heads into the jungle to find her, Jack, Sayid and Charlie have no choice but to follow. Meanwhile, Locke asks Claire to help build a mysterious item.
Locke and Boone build a trebuchet in an attempt to open the hatch, but Locke is injured when the trebuchet fails. Literally following a hallucination, Locke and Boone discover a small plane crashed high in a tree. Sawyer, suffering from severe headaches, is forced to turn to Jack for help.
Jack suspects foul play when Michael becomes violently ill while building the raft. Meanwhile a secret from Kate's past is revealed, the mysterious hatch is shown to a few of the survivors, and Walt gives Locke a warning.
Danielle Rousseau arrives in the camp of survivors at the beach and advises that "The Others" are coming. There is black smoke in the sky, and they have three choices: run, hide, or die. Jack recalls his meeting with Ana-Lucia Cortez in the airport. Meanwhile, the group is about to launch the raft. Sawyer tells Jack the conversation he had with his father in a bar in Australia. Jack, Kate, Locke, Hurley and Leslie Arzt go on an expedition to bring explosives from the Black Rock to blow the hatch. There they can shelter the survivors against "The Others". Meanwhile, ...
(Part II of II) The castaways on the raft are surprised at sea by something unexpected. Meanwhile, remaining islanders attempt to blow open the hatch, and a visitor to the encampment might be a threat to Claire's infant son.