Two outcomes of the detonation of the hydrogen bomb are presented. Flight 815 lands safely at LAX, where Kate escapes from the marshal and Jack learns that his father’s body was lost in transit.
After Sun is attacked, some of the survivors begin to think that one of the “other survivors” might be one of “The Others.” When French woman Danielle Rousseau leads Sayid to a mysterious captive in the jungle, he becomes determined to find out if he is one of “The Others”. Meanwhile, Sawyer discovers Hurley’s potentially devastating breach of the survivors’ trust and blackmails him into helping track an elusive island creature that won’t leave Sawyer alone.
For different reasons, Jack and the Man in Black both use Desmond to extinguish the light at the heart of the island. The island starts disintegrating, but The Man in Black is also made mortal. Still desperate to escape, The Man in Black fights Jack and mortally wounds him, but with help from Kate, Jack kills him. Realizing his destiny, Jack sacrifices himself to relight the heart of the island, so his friends can escape on the Ajira plane. Hurley takes Jack’s place as protector of the island and makes Ben the new advisor. The flash sideways is revealed to be a type of limbo, or holding pattern, for the main characters, where they reside until accepting their life and death, and are then able to “let go” and “move on” together.
Jack and Kate find themselves at odds over the direction to take to save their fellow island survivors, Locke further solidifies his stance as leader of “The Others,” and Sawyer and Juliet come under scrutiny from the Dharma Initiative.
Locke thinks he’s being sent a sign on how to get the hatch open, and he and Boone venture inland. Jack is reluctant to offer assistance when Sawyer begins to experience excruciating headaches and needs glasses.
The survivors are jolted awake in the middle of the night when wild island beasts (which are wild boars) invade the beach encampment. Kate and Michael join the mysterious Locke on a hunt for food — and a shocking secret about Locke is revealed. On the hunt for food someone is injured. Meanwhile, some survivors are horrified by Jack’s plan for the dead bodies still scattered among the wreckage — he wants to burn them. Jack sees someone that’s not there, and we find out that one of the survivors was not able to walk but now he can.
With the abduction of Walt fresh on their minds, their raft destroyed, and Jin missing, Michael and Sawyer fight for their lives in the middle of nowhere in the ocean and discover a new predator in the roiling sea. Meanwhile on land, Locke must descend into the hatch when Kate goes missing inside. Also, Jack isn’t too far behind Locke as he decides to go into the hatch as well later on. Lastly, flashbacks reveal more of Michael’s troubled relationship with ex-lover Susan and their baby son Walt as they fight over custody for Walt and Michael must let go.
Claire has a horribly realistic nightmare about her new baby being harmed or kidnapped. Flashbacks reveal Claire’s backstory: the former boyfriend who got her pregnant, then abandoned her, and the psychic who convinced her to take the ill-fated flight that landed her on the island. Hurley is shocked and confused when he discovers that Ethan Rom (guest-star William Mapother), another plane crash survivor, does not appear on the flight manifest.
The face-off between the survivors and the freighter people continues, and the Oceanic Six find themselves closer to rescue.
Hurley struggles with an assigned task inside the hatch as he flashbacks to disturbing memories in his life before the crash. Meanwhile, Sawyer, Michael and Jin learn the identities of their captors on the island. Claire uncovers a startling piece of information about the fate of the raft.
Sawyer’s wound becomes life-threatening as he, Michael and Jin make their way through the interior of the island with the tail section survivors. Meanwhile, Shannon is once again haunted by visions of Walt.
Ben unwillingly divulges information to Locke about the island, taking him on a journey to various locations including strange monuments and the mysterious Jacob. Back at the beach, Juliet’s secret is revealed. Flashbacks will show us the origins of DHARMA and the history of the island, including ‘the purge.’
When Charlie’s vividly surreal dreams lead him to believe Claire’s baby, Aaron, is in danger, Locke suspects Charlie may be using again. Meanwhile, Sawyer encourages Hurley to act on his attraction to Libby.
Locke’s mission off the island as Jeremy is revealed.
Sayid goes back to the island to bring the rest of the people to the freighter. Ben, Locke, and Hurley go to another Dharma station in order to “move the island”.
Ben finally takes his revenge on Widmore after accepting the Man in Black’s request for assistance. Jacob explains the purpose of the candidates, and Jack volunteers to take his place. The search for Desmond reveals that he has been rescued and the Man in Black plans to use him to destroy the island. In the flash sideways, each person ends up on their way to a concert.
On the 100th episode milestone for the series, the time of reckoning has begun when Daniel Faraday comes clean regarding what he knows about the island.
Kate needs to get information out of the hostages, but it may jeopardize her standing with Locke — as well as with Sawyer. Meanwhile in the future Kate’s Court hearings take place.