Catherine's frustration intensifies when a lead suspect in Grace's disappearance is found murdered; the team exhausts all their skills to figure out who killed the suspect, and if his death will lead them to Grace.
Desmond looks for a woman who might be the key to helping Faraday stop the island's unpredictable movements through time; Locke finds out who has been attacking the survivors.
All of Coulson’s secrets and lies come to a head when the team discover murder victims with the alien glyphs on them that he’s been etching.