A parallel universe, in which the crash never happened, is revealed as an aftermath of the bomb's detonation. Meanwhile, Sawyer blames Jack for the loss of Juliet and the true identity of the monster is revealed.
Jack, Sawyer, Kate, Miles and Hurley, struggling to save Sayid, face an ancient secret. Meanwhile, a fake Lock exits the chamber to everyone's awe.
Kate finds herself on the run, while Jack is tasked with something that could put a friend's life at risk.
The John Locke impostor takes Sawyer on a trek through the jungle towards answers, while in the alternate reality John Locke struggles with life on a wheelchair.
Hurley asks Jack to come with him at the behest of Jacob, while Jin is forced to watch as Claire interrogates Justin about the location of her son. Meanwhile, in another life, Jack meets the son he never knew.
Dogen asks Sayid to make a decision, while Locke sends Claire to give the temple inhabitants a warning. Meanwhile, in another life, Sayid must come to the aid of his family and meets a familiar face.
Ilana discovers the extent of Ben's involvement in Jacob's death, while Hurley and Jack meet Richard. In another life, Dr. Ben Linus, a high school history teacher, must make an important choice.
Locke sends Sawyer to Hydra Island to investigate the plane, but he encounters Widmore's people instead. Meanwhile, in another life, Sawyer is a police detective looking for Anthony Cooper, the man responsible for his father's death.
Richard Alpert faces a difficult choice, and remembers how he came to the island... over a hundred years ago.
Widmore's people abduct Jin, and Locke vows to get him back. Meanwhile, in another life, Jin and Sun come together but Sun's father seeks to separate them... permanently.
Widmore has Desmond brought to the island for an experiment... one that will allow Desmond to visit the alternate reality.
Hurley tries to determine what he has to do to save everyone, and who to listen to. Meanwhile, Locke meets Desmond, Richard insists on blowing up the plane, and in another life, a rich and successful Hurley meets a woman from an insane asylum who insists she knows him.
As alliances are forged and broken as the Locke and Jack camps merge. Widmore delivers his ultimatum. Meanwhile, Jack makes a decision, Jim and Sun are reunited, and in another life, the candidates draw ever closer together.
After the remaining survivors are reunited, Sawyer and Jack hatch a plan to divert the Man in Black's attention and leave the island without him on Widmore's submarine, but disastrous consequences await them. In the flash sideways, Jack investigates the cause of Locke's paralysis and offers treatment.
Jacob and the Man in Black are revealed to be twin brothers. They are raised by a mysterious woman who is charged with protecting the island. As revenge for the hug of their adoptive mother (who killed their real mother), Jacob throws his brother into the source of electromagnetism after being made the island's guardian, which transforms the Man in Black into the smoke monster.
Locke gains a new follower and goes to confront Widmore. Meanwhile, Jacob meets with the candidates and one of them must choose their destiny. And in another life, Locke makes a decision while Desmond gathers the others.
Jack, the new protector of the island, meets with Locke and they go past the bamboo field with Desmond to visit the heart of the island. The Man in Black's final fate is determined, and one survivor must make the ultimate sacrifice to save the island. Meanwhile, in another life, the ones who might have been the survivors gather for a final time.
The Man in Black's final fate is determined, and one survivor must make the ultimate sacrifice to save the island. Meanwhile, in another life, the ones who might have been the survivors gather for a final time.