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.

A mysterious illness that baby Aaron suddenly contracts sends a desperate Claire, Kate and French woman Danielle to return to the same area where Claire was initially kidnapped. It is there where she believes she can find a cure for Aaron's illness. In the meantime, Jack and Locke are doing their best to keep their prisoner a secret from the rest of the survivors on the island.

Sun comes to the realization that she might be pregnant. She struggles on whether to tell Jin about the situation. Meanwhile, with no success from Jack or Sayid, Ana-Lucia is called in by Locke to interrogate the new prisoner Henry Gale to get more information out of him about the balloon that he and his wife supposedly landed on the island with.

When the hatch suddenly takes on a life of its own, Locke is forced to enlist the help of an unlikely ally to control the matter. Meanwhile, Ana Lucia, Sayid and Charlie go off into the jungle to find out the truth about Henry Gale and see if there really is balloon craft out there in the jungle that carried him and his wife to the island.

Libby lends a helping hand to Hurley to support him when he begins to think the island is having a strange effect on him. Hurley begins to see Dave (guest-star Evan Handler, Sex and the City) on the island, who was his friend in the mental institution Hurley was in. Also, Locke's sense of purpose is shaken when the prisoner gives him new information about the hatch.

Rose is surprisingly and vehemently opposed to Bernard's plan to create an S.O.S. signal; romantic sparks are rekindled between Jack and Kate when they trek into the jungle to propose a "trade" with "The Others"; and Locke begins to question his faith in the island.

After finding an exhausted Michael in the forest, Jack and Kate bring him back to the main camp. When he finally wakes up, Michael has some new details about "The Others." Also, a lovestruck Hurley plans a date for Libby.

After discovering something odd just offshore, Jack and Sayid come up with a plan to confront "The Others" and hopefully get Walt back. At the same time there will be an answer to the question of where Michael has been and resolution of him and Walt. Meanwhile, Eko and Locke come to blows as Locke makes a potentially cataclysmic decision regarding the "button" and the hatch. Lastly, Desmond returns and he sheds some more light on his experience on the island in the three years prior to when Locke came down into that hatch.