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 while trying to find a way to be rescued.
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.
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.
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.
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.
(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.