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 I 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.
Kate's original crime that started her life on the run is revealed. Meanwhile, the survivors lay one of their own to rest, Kate sleeplessly watches over a feverish Sawyer, and Mr. Eko has a surprise for Locke regarding the hatch.
When Charlie's vividly surreal dreams lead him to believe Claire's baby, Aaron, is in danger, Locke suspects Charlie may be using drugs again. More of Charlie's background story shows his difficult relationship with his heroin addicted brother. Meanwhile, Sawyer encourages Hurley to act on his attraction to Libby.
Survivors fear that "The Others" may have returned when Sun is injured during a failed kidnapping attempt. Meanwhile, Sawyer is an amused, but highly interested, bystander when tension escalates between Jack, Locke, Kate and Ana Lucia over control of the hatch computer, and the storage of the weapons.
The mysterious French woman, Danielle Rousseau returns to lead Sayid to a mysterious captive who claims he crashed on the island. Meanwhile, Sawyer recruits Hurley to help him find another island animal that is harassing him: a loud tree frog.
When Claire's baby falls ill, she becomes determined to find out what happened to her to regain her memory during her captivity with the Others, triggering flashbacks to her experience among the Others, and her ordeals concerning her baby. Also, Mr. Eko learns about Henry Gale and asks Jack and Locke to give him a meeting with their captive.
When Sun comes to the realization that she might be pregnant, she struggles on whether to tell Jin about the situation. After no success from Jack or Sayid, Ana Lucia is called in by Locke to interrogate the prisoner, Henry Gale, to get more information out of him about how he arrived on the island. Ana Lucia, Sayid, and Charlie then set out into the jungle to try and confirm Henry's story.
When the hatch suddenly takes on a life of its own, Locke is trapped alone in the bunker and is forced to enlist the help of an unlikely ally: the captive Henry Gale to try to free him. Meanwhile, Jack faces off against Sawyer in a series of card games for supplies. Also, Ana Lucia, Sayid and Charlie find out the truth about Henry in the jungle.
Hurley begins to think the island is affecting him in a very strange way, so Libby attempts to offer him support. Hurley also recalls his past in a mental hospital with his friend Dave. Meanwhile, the prisoner calling himself Henry Gale offers new information about the Hatch, which severely shakes Locke's sense of purpose.
Jack and Kate bring an exhausted Michael back to camp where he has news about "The Others". Meanwhile, Ana Lucia attempts to get the prisoner, Henry Gale to confess to being a spy for the Others, and he attempts to kill her. Also, Hurley plans a surprise date for Libby.
Mr. Eko enlists Locke to help find a secret location he believes houses answers to the island's mysteries. Meanwhile, Jack, Sawyer, and Kate struggle to cope with the horrific situation in the hatch following the escape of "Other" Henry Gale, and of the shooting deaths of Ana-Lucia and Libby, unaware that it was Michael who is responsible.
Michael's whereabouts during his 13 days in captivity in the "Others" camp are detailed while in the present, he tries to convince Jack to help him mount a search party for his son Walt still being held captive. Meanwhile, Locke struggles with a choice whether or not to abandon the hatch.
After discovering a sailboat just offshore containing former hatch oversee Desmond, Jack and Sayid come up with a plan to confront "The Others" and hopefully get Walt back. Meanwhile, Eko and Locke come to blows as Locke makes a potentially cataclysmic decision regarding the "button" and the hatch.
Having returned to camp, Sawyer's help is enlisted by Hugo, who has an obsession with retrieving an old VW van found on the island. More events in Hugo's life are also revealed, showing what brought him to be on the doomed flight.
Claire launches a plan to use migratory birds to send a message, but she doesn't understand why Desmond keeps sabotaging it. Elsewhere on the island, Sayid, Locke and Kate scale The Others' protective perimeter and discover their camp.
Desmond has another vision of Charlie's death, but this time his death ultimately will result in Claire's rescue. Meanwhile, the survivors discover that an attack by the others is even more imminent than originally expected.
Jack and the survivors wait for Charlie to turn off The Others' signal-jamming device, not knowing that he has been captured at the Looking Glass station. In addition, Sayid, Jin and Bernard are captured after The Others invade the camp.
Charlie's dying message casts doubt on the intentions of the people that Jack believes to be their rescuers. A power struggle ensues between Jack, who wants to leave the island, and Locke, who thinks that they're supposed to stay.
The surviving members of the supposed "rescue team" are scattered around the island after a helicopter problem. However, once they are located, they reluctantly reveal that they are indeed not there to rescue them.
Sayid negotiates a deal with Locke to hand over Charlotte from the "rescue team," so that he can get on their helicopter and reach their ship. Kate joins Sayid for the mission, but she decides to stay behind with Sawyer.
Kate goes behind Locke's back to arrange a meeting between Miles and Ben, where Miles offers his terms for not giving Ben up to his colleagues. However, Locke discovers what she has done, and banishes her from his camp.
During the helicopter ride to the ship, Desmond's consciousness begins jumping back and forth between present day 2004 and 1996. He is forced to find Daniel Faraday in 1996 in order to get answers and stop the jumps.
Daniel and Charlotte leave Jack's camp to fulfill their objective, and Jack and Juliet try to track them down before they are successful. Juliet's romantic history on the island with The Others is exposed.
Michael's journey from escaping the island to working as Ben's spy on Widmore's ship is revealed, as well as his mission. Ben tells Danielle and Alex to leave Locke's camp and go to the island temple, but they are ambushed on the way.
The team from the ship attacks Locke's camp looking for Ben, with Alex as a hostage. Meanwhile, the doctor from the ship washes up on shore at Jack's camp, and Daniel Faraday provides some distressing news to Jack.
Ben, Locke, and Hurley go to The Orchid, but Keamy and his men beat them there. Jack and Kate go after the helicopter, and are later joined by Sawyer. Sayid comes back to the island with a rescue boat, and then goes after them too.
Ben tries to escape Keamy's custody so that he and Locke can follow through with Jacob's request to move the island. Michael tries to neutralize the explosives on the boat, while the "Oceanic 6" move closer to rescue.
As a result of Ben moving the island, the survivors left on the island begin skipping through time. Meanwhile, three years later in Los Angeles, Ben starts convincing the Oceanic 6 to return to the island.
Desmond returns to Oxford to fulfill Faraday's request to find his mother. Back on the island, the survivors discover that they've run into The Others 50 years in the past, but Richard Alpert is the same age.
John Locke, now alive again and back on the island after the second plane crash, flashes back to his return to civilization, where he is brought in by Charles Widmore and asked to convince the Oceanic 6 to return to the island.
Jack, Kate and Hurley wind up on the island in 1977, where they meet up with Sawyer and also join the Dharma Initiative. Meanwhile, Sun is stuck on the island in the present with Ben and the other passengers after the plane crash.
Jack refuses to help save young Ben after Sayid shoots him, so Sawyer and Kate turn to Richard Alpert for help. Meanwhile, what Sawyer said to Kate on the chopper is revealed, as well as what Kate did with Aaron before the flight.
Ben and Locke return to the island, where Ben says he is going to summon the Smoke "monster" and atone for his daughter's death. In addition, the falling-out on the island between Ben and Charles Widmore is revealed.
Miles is chosen by Horace to transport a dead body across the island. During the trip, Miles reveals to Hurley that Dr. Pierre Chang (a.k.a. Dr. Marvin Candle) is his father, whom he resents for leaving him and his mother.
Jack attempts to follow through with Daniel Faraday's plan to detonate the hydrogen bomb after Faraday is killed, Locke instructs Alpert to take him to Jacob, and Sawyer is interrogated about an incriminating surveillance video.
Jack and Sayid's plan faces resistance from Sawyer, Juliet, and Kate. Ben and The Others/Hostiles reach Jacob at the Foot of the Statue, and are soon met by the 316 survivors, who reveal the contents of their mysterious package.
Jacob sends Hurley on a mission across the island, but Jack must accompany him. In Los Angeles, Jack's difficult relationship with his son is revealed. Meanwhile, Jin discovers that Claire thinks the people at the temple stole her son.
Claire arrives at the temple and warns the inhabitants that the being inhabiting Locke wants to talk, but Dogen sends Sayid to kill him instead. In Los Angeles, Sayid saves his brother, who is in trouble with a loan shark.
Ilana prepares to kill Ben after Miles reveals that Ben killed Jacob. In Los Angeles, Ben contemplates a power play at his high school. Meanwhile Richard and Jack share their secrets and experiences with Jacob and form an alliance.
Locke sends Sawyer on a recon mission to the other island, where he runs into Charles Widmore and his submarine crew. In Los Angeles, Sawyer works as a police officer obsessed with finding the conman who destroyed his family.
The mysteries behind Richard Alpert's arrival to the island, his immortality, and his alliance with Jacob are revealed--plus more details about the purpose of the island and the relationship between Jacob and the smoke monster.
Sun and Jin's search for each other is interrupted when Widmore's crew kidnaps Jin, and Locke heads to the other island to bring him back. In Los Angeles, Jin's job for Sun's father is sidetracked when his money is taken at customs.
Charles Widmore brings Desmond back to the island so that he can conduct a dangerous electromagnetic experiment, which sends him back to the alternate reality in Los Angeles, a reality where he has never met Penny.
With Jack's help, Locke rescues the candidates from Widmore's compound, but their plans for leaving the island are forced to change. In Los Angeles, Jack tries to convince Locke to try an experimental surgery for his paralysis.
Locke searches for Desmond to achieve his goal of destroying the island. Jack, the newly-appointed "protector" of the island, sets out to stop him. In Los Angeles, the survivors slowly reunite and recall their island experiences.