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.

The castaways on the raft are surprised at sea by something they didn't expect. Meanwhile, remaining islanders try to blow open the hatch, and a visitor to the encampment might be a threat to Claire's infant son.