When an air flight veers off-course and ends up crashing on a tropical island, the survivors struggle to figure out a way to survive as the island appears to hold more danger than what meets the eye.
As a number of survivors try to send out a distress signal, they come to learn more about survivors from other incidents left stranded on the island. A disturbing truth is revealed about Kate's history.
As Jack learns more about Kate, he and Sawyer clash over the fate of the marshal, forcing Jack to take drastic measures to combat Sawyer's mistake.
As their provisions run out, a party of survivors goes on a hunting trip to secure more food, only for Locke to run into the mysterious island monster.
Weighed down by exhaustion and the weight of responsibility, Jack hallucinates about his late father, with his attempt to catch up with his father leading to a discovery that could mean life for the stranded survivors.
As the survivors split into two groups, an altercation between Jin and Michael reveals more about the former's complicated past and his tense relationship with his wife, Sun.
As Jack's life hangs in the balance after a cave-in, Charlie is forced to reevaluate his life choices after he starts going through withdrawal.
When Sawyer stands suspected of stealing Shannon's inhaler, Sayid takes drastic measures to figure out the truth. Kate finds out the disturbing details of Sawyer's childhood.
As Sayid stumbles upon the source of the distress signal, he learns more about the other inhabitants of the island and the hidden dangers that lurk everywhere.
As the survivors become aware of Rousseau and the other island inhabitants, Claire goes through an experience that makes everyone suspect that they have a mole in their midst.
With Claire and Charlie held hostage, a group of four goes looking for them. But as Locke and Boone encounter a mystery, Jack and Kate are given a warning to back off.
When Kate stumbles upon the marshal's case, her desperation to retrieve an item from it reveals more about her past. Rose tries to rouse a depressed Charlie.
As Shannon and Sayid get close, Locke takes measures to help sever Boone's connection to Shannon as he becomes determined to tell her about the hatch they found.
As Walt wanders off, Michael and Locke join forces to save him as he lands himself in peril. Boone and Locke stumble upon a surprising discovery.
As the survivors try to come to grips with Claire's return and her apparent amnesia, a plan is formed to lure Ethan in after he delivers a threat to Charlie.
After becoming obsessed with finding the boar that got into his tent, Sawyer takes a journey into the jungle that forces him to reflect on a deadly mistake from his past.
When Michael's raft burns down, Sun reveals that she can speak English as she steps in to save Jin from Michael's accusations. Jin and Sun's relationship hangs on by a thread.
When Hurley discovers an unexpected connection between Rousseau and a significant number from his past, he sets out to figure out the truth behind it, only to find more questions than answers.
Locke grows desperate when he starts losing the ability to walk. Boone makes an unsettling discovery just before getting critically injured.
With Boone's life hanging in the balance, Jack pulls out all the stops to save him. Charlie finds himself in an unexpected predicament when Claire goes into labor.
In the wake of Boone's death, Locke becomes an outcast among the survivors as Jack confronts him about his lies, and Shannon blames him for Boone's death.
Tempers flare as the survivors try to figure out who tried to poison one of their own. Walt tries to convince Locke not to open the hatch.
The survivors gain an unlikely ally in the form of Rousseau, who warns them about a possible attack from the group known as the Others. Michael sets sail on the raft.
When Rousseau unexpectedly turns on the survivors, tension rises as they try to balance their search for her, their preparation for the Others, and their confrontation with the island monster.
As the truth about Rousseau's unexpected betrayal is revealed, the survivors on the raft face mortal danger as their confrontation with another boat leaves several lives hanging in the balance.